By Using polymethyl methacrylate （PMMA） as the high-molecular polymer substrate and SiO2 as the filler，a kind of composite fiber with daytime radiative cooling function was prepared via electrospinning.Then，the XRD，SEM，TEM，UV-Vis-NIR and FT-IR were used to characterize the morphology，structure and optical properties of the composite fiber.The results show that the diameter of the electrospun composite fiber is about 800nm in diameter.The forbidden band width of pure PMMA fiber is 4.95eV，which makes the fiber do not absorb visible light; and the orientation crystallization in the electrospinning process leads to crystalline region and amorphous region in the fiber，which does not transmit visible light and displays white，thus obtaining a 200~2500nm short-wavelength reflectivity as high as 97.57%.After the electrospinning，nano-SiO2 powder particles with appropriate amount are dispersed in the composite fiber，which can greatly improve the emissivity.When the doping amount of SiO2 is 7.5%，the reflectivity of the composite fiber in the solar short-wavelength band of 200~2500nm and the emissivity of the 8~13μm atmospheric window are respectively up to 96.53% and 0.903.By using a self-made device to test the cooling effect in the actual environment，it is found that the composite fiber behaves good cooling effect，the temperature of the back of the composite fiber in the daytime is lower than the ambient temperature by 2~4℃.The proposed preparation method of composite fiber with inorganic powder and organic matrix provides a new solution to the practical application of radiative cooling.