Objective: Rigid polyurethane foam (RPUF) is widely used in building insulation and other applications, yet its inherent flammability and substantial release of toxic smoke during combustion severely limit its safe use across various scenarios. Conventional flame-retardant strategies often cause environmental pollution and impair mechanical properties, whereas bio-based flame retardants alone exhibit limited effectiveness. Herein, based on the intumescent flame-retardant theory, a synergistic flame-retardant system was constructed using melamine (MEL) as the gas source and silk fibroin (SF) as the charring agent. This study systematically examines the effects of different MEL/SF mass ratios on the pyrolysis, combustion, and smoke-toxicity characteristics, and elucidates the underlying synergistic mechanism to provide a design reference for developing high-efficiency, low-toxicity bio-based flame-retardant RPUF. Methods: MEL/SF/RPUF composites with MEL/SF mass ratios of 0, 5∶2, 5∶1, 15∶2, and 10∶1 were fabricated via a one-step foaming method at a total additive loading of 10 wt%. Thermogravimetric analysis was used to examine the thermal decomposition behavior. Apparent activation energies were calculated via the Friedman differential and Starink integral methods, and the Malek method was applied to identify the most probable reaction mechanisms. Cone calorimetry was conducted to record the heat release rate, smoke production, and CO yield, from which the fire growth index and toxicity potential index were derived to quantify fire hazards. The residual chars were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy to determine their micromorphology, elemental composition, and graphitization degree. Thermogravimetry–infrared spectrometry was additionally employed to identify the gaseous pyrolysis products. Results: The pyrolysis process of MEL/SF/RPUF composites can be divided into three stages. Increasing MEL content lowered the onset decomposition temperature, and the apparent activation energy followed a three-stage trend of "rising–fluctuating–high-level stabilization." The MEL/SF/RPUF-4 sample (MEL/SF = 15∶2) consistently showed the highest average activation energies across all stages, with final-stage values reaching 147.59 kJ·mol⁻¹ (Friedman method) and 140.84 kJ·mol⁻¹ (Starink method), indicating superior thermal stability. As MEL content increased, the reaction mechanism in the first half of pyrolysis gradually transitioned from the F2 model to the A2 model, whereas the second half deviated from all theoretical models, exhibiting complex mixed-control kinetics. At an MEL/SF ratio of 15∶2, the composite achieved the lowest mass loss rate and the highest residual mass (21.58%), which was 1.5 times that of the SF-only modified group. The peak heat release rate decreased by 23.36%, the peak smoke release rate dropped by 63.69%, and both the fire growth index and the toxicity potential index reached their minimum values. The char layer at this ratio was the most compact, smooth, and highly graphitized, with a carbon content of 75.9% and nitrogen retention of 11.3%. The early decomposition of MEL released NH₃, which diluted the gas phase while promoting char formation in SF, demonstrating a pronounced gas–solid synergistic flame-retardant mechanism. Conclusions: The MEL/SF system exhibits a pronounced gas–solid synergistic flame-retardant mechanism, with the MEL/SF ratio of 15∶2 identified as the optimal formulation. The composite at this ratio achieves the best overall performance in thermal stability, heat suppression, smoke reduction, and toxicity mitigation, offering a design reference for developing high-performance, environmentally friendly bio-based flame-retardant RPUF.