Objective: Transmission towers are essential components of power transmission systems, and their safety directly affects grid stability. With the increasing frequency of extreme weather events, damage from combined wind-rain loads has become more pronounced. Current design codes primarily address wind loads in isolation, neglecting the effects of wind-rain coupling. In addition, the effects of key factors, such as wind direction, on structural response remain unclear. This study investigates the displacement response of transmission tower-line systems under wind-rain coupling and clarifies how wind speed, rainfall intensity, wind direction angle, and measurement location influence this response. The goal is to provide reliable experimental data to optimize tower design against combined wind and rain loads. Methods: A 1:10-scaled aeroelastic model of a 110 kV cat-head transmission tower was designed and constructed based on similarity criteria. Comprehensive wind tunnel tests were conducted in a large-scale climate wind tunnel while considering three wind speeds (10.0, 14.0, and 18.0 m/s), three rainfall intensities (30, 60, and 90 mm/h), and three wind direction angles (45°, 60°, and 90°). A finite element model of the prototype tower was developed using advanced simulation software tools for modal analysis. A high-stiffness sensor support was designed to minimize vibration interference. Two laser displacement sensors were used to measure the root-mean-square (RMS) and peak displacements at the upper and lower sections (Measuring Points 1 and 2, respectively) of the tower top in a synchronous manner. Key similarity parameters, including length, wind speed, frequency, and stiffness, were strictly controlled to ensure the validity of the test results. Results: The experimental results yielded four key insights: (1) Wind speed had a non-linear strengthening effect on displacement response, with a significantly greater growth rate in the high wind speed range (14.0-18.0 m/s) than in the low wind-speed range (10.0-14.0 m/s). (2) Displacement at measuring point 1 consistently exceeded that at measuring point 2, indicating that the upper section of the tower top is the most vulnerable region under wind-rain coupling. (3) The 45° wind direction angle was identified as the most critical, producing a significantly larger displacement response than other angles. (4) A critical coupling effect between wind and rain was observed, with moderate increases in displacement response at lower rain intensities (≤60 mm/h) and wind speeds (≤14.0 m/s). Under extreme conditions (90 mm/h rain intensity and 18 m/s wind speed), the RMS displacement at measuring point 1 (45° wind angle) increased significantly compared with that under moderate conditions. Conclusions: This study systematically elucidated the displacement response characteristics of transmission towers under combined wind and rain loads, quantitatively assessing the influence of key environmental and structural factors. The results suggest that the upper section of the tower top should be prioritized for reinforcement in design. The combination of a 45° wind direction angle with extreme wind and rain conditions (18.0 m/s, 90 mm/h) constitutes a critical design scenario. In addition, wind direction angles of 60° and 90° can be classified as a "low-response group" allowing for potential design optimization. These findings provide a crucial experimental basis for advancing transmission tower design theory for combined wind and rain conditions, effectively balancing structural safety with economic efficiency.