The standard fire resistance test failed to consider the effect of environmental wind on intumescent fire-retardant coatings. This study investigated the thermal insulation of intumescent fire-retardant coatings under various heating exposures (25 kW/m², 35 kW/m², 50 kW/m² and 75 kW/m²) and wind speeds (0 m/s, 1 m/s, 2 m/s, 3 m/s). Fire-retardant coating presented a non-uniform intumescent process with wind influence. Environmental wind slowed the initial swelling rate, decreased the maximum coating thickness, and promoted the coating thickness decline. The coating at the front end was shorter than the rear end, which caused a higher steel temperature at the front end. The time of the steel temperature to reach the critical temperature was advanced indicating a decreased thermal insulation. Environmental wind caused a higher coating temperature and accelerated the coating depletion at the front end. The influence is stronger with higher heating exposure and faster wind speed. Wind affects the coating temperature and coating thickness via convective cooling and char oxidation, and char oxidation is a more dominant factor. Empirical correlations were applied to represent the influence of environmental wind on the maximum swelled thicknesses and average swelling rates of fire-retardant coatings at the front end. The predicting equation indicated the synergistic effects of wind speed and incident heat flux on the thermal insulation of fire-retardant coating.