The determination of quantum efficiency of a photodetector is of great importance in radiometry. External quantum efficiency of windowless silicon photodetectors is measured with cryogenic radiometer at ten wavelengths of He-Ne, Ar+-Kr+ and Tisapphire lasers. Based upon measured reflectance at three laser wavelengths, the thickness of the silicon dioxide layer is obtained through least square method according to the relation between surface reflectance of a photodetector and the thickness of its silicon dioxide layer. Spectral reflectance of the photodetector surface is thereby obtained. Internal quantum efficiency is calculated from surface reflectance and external quantum efficiency result at the laser wavelengths, and spectral quantum efficiency is fitted. The deviation of modeled result from experimental result is within 1.5×10-4 in the spectral range from 488 to 900 nm.