[Objective] The simultaneous occurrence of fire-related smoke and combustible gas leakage, particularly methane, poses a significant and complex threat in various environments, including residential kitchens, commercial catering establishments, and industrial workshops. Conventional detection systems are typically designed to monitor a single hazard type, requiring multiple sensors for comprehensive risk coverage. This research aimed to develop a novel, highly integrated, and cost-effective composite detector that can simultaneously monitor smoke and methane. [Methods] The foundation of our proposed method is a composite detection model utilizing the optical properties of a single 1 653.7 nm laser. This wavelength was specifically chosen because it aligns with a characteristic absorption line for methane and is capable of inducing Mie scattering from smoke particles. The main methodological challenge was to decouple the composite signals, as methane absorption and smoke extinction attenuate the transmitted laser intensity. To achieve this, a sophisticated signal processing strategy was developed. We employed tunable diode laser absorption spectroscopy in a time-division multiplexing scheme using a single triangular wave scan. The rising edge of the scan incorporated high-frequency harmonic modulation, enabling wavelength-modulation spectroscopy (WMS) to extract the second-harmonic signal. By contrast, the falling edge of the scan remained unmodulated, allowing for direct absorption spectroscopy (DAS) to provide robust measurements for higher methane concentrations. The decoupling of the smoke signal was achieved by analyzing the baseline of the scattered light signal, which corresponds to wavelengths where methane absorption is negligible, making its intensity directly proportional to smoke particle concentration. In addition, to mitigate the effects of smoke on methane measurements, all absorption signals were normalized during processing. To further enhance the detector’s performance, a novel optical structure was designed to address the inherent weaknesses of the 1 653 nm laser, specifically, the relatively weak scattering signal from smoke particles and the need for a long optical path for sensitive methane detection. We implemented a multimirror, “cross-beam” optical path within a compact chamber, which effectively folds the laser beam, causing it to traverse the detection volume multiple times before reaching the photodetector. This innovative configuration resulted in a sixfold extension of the effective absorption path length for methane, increasing it from the standard 10 cm to 60 cm. By optimizing the placement of the mirrors and the angle of the scattering detector, we configured the system to collect scattered light from high-intensity beam-crossing regions, leading to a fourfold enhancement of the collected smoke scattering signal. [Results] In single-analyte tests, the detector exhibited high sensitivity, achieving a lower detection limit of 0.008% volume fraction for methane. The hybrid WMS-DAS approach proved effective, with WMS yielding more accurate results for concentrations below 0.25%. A linear regression analysis of the measurement data indicated excellent linearity (R²=0.9833). For smoke detection, the device achieved a detection limit of 0.05 dB·m^-1. The critical validation was the composite detection experiment, in which methane detection was performed under various stable background smoke densities (0, 0.1, 0.5, and 1.0 dB·m^-1). The results conclusively demonstrated the effectiveness of the signal decoupling methodology, showing that the presence of smoke, even at high densities, did not significantly affect the accuracy of methane concentration measurements. However, at the highest smoke densities (0.5 and 1.0 dB·m^-1), the WMS signal exhibited increased fluctuations, indicating that extreme smoke levels can degrade the signal-to-noise ratio, setting a performance boundary for the current system. [Conclusions] This study proposed and validated a novel method for the synchronous detection of smoke and methane using a single 1 653 nm laser source. A functional prototype of the composite detector demonstrated the feasibility and effectiveness of the approach. Major innovations include a composite detection model that enables signal decoupling through a time-division WMS/DAS scheme and a performance-enhancing cross-beam optical path that extends the absorption length sixfold and enhances the scattering signal fourfold. Most importantly, it demonstrated robust performance in composite scenarios with minimal cross-interference. By overcoming the limitations of conventional systems, this single-source composite detection method offers a promising, low-cost, and highly integrated solution, providing a new technical pathway for advanced early warning systems in complex, multihazard environments.