Abstract: To address the limitations of existing passive radiative cooling materials, which are primarily designed for near-ambient conditions and are susceptible to enhanced parasitic solar absorption, intensified atmospheric back-radiation, and thermo-oxidative degradation of polymers at elevated temperatures of approximately 400℃, a surface-roughened polyimide (PI) /TiO₂ microsphere composite film was developed. Its high-temperature radiative cooling performance was investigated through finite-difference time-domain (FDTD) simulations and experimental measurements. The results show that the surface roughness and TiO₂ microspheres jointly enhance solar-band reflection while modulating mid-infrared absorption/emission through changes in the local electromagnetic field distribution and effective optical path length. The fabricated composite film exhibits a hemispherical solar reflectance of approximately 0.90 and an average emissivity of approximately 0.94-0.95 over the 2.5-25 μm range. Under the calculation condition of T=T_amb=27℃, the net radiative cooling power reaches 113 W·m⁻². Outdoor near-ambient tests demonstrate a daytime sub-ambient temperature reduction of approximately 3 K. When the heating stage is maintained near 400℃ under identical substrate and enclosed wind-shielding conditions, the surface temperature of the PI-TiO₂ composite film is more than 40 K lower than that of the neat PI control film. This high-temperature temperature difference represents a system-level response resulting from the combined effects of conduction, convection, solar absorption, and mid-infrared radiative heat exchange. Short-term thermal treatment and thermogravimetric measurements indicate good initial thermal stability of the composite film; however, its long-term durability against dust deposition, water-film formation, hygrothermal exposure, and ultraviolet weathering requires further investigation.