MOCVD epitaxial P-GaN, which was grown on the sapphire substrate and AlN buffering layer, was used to make photocathode by ultra-high vacuum preparation technique. The quantum efficiency properties of prepared GaN photocathode under reflection mode and the transmission mode measured and analyzed by UV spectral response testing instrument. Experimental results showed that the sample in reflection mode has up to 50% quantum efficiency at 240 nm, but in transmission mode, the maximum quantum efficiency is only 15%. Curve fitting results of experimental quantum efficiency revealed that sample′s back interface recombination velocity is about 104 cm/s. It was also found that the reasons for the great difference in this two operating modes were back-interface recombination velocity and the thickness of GaN epitaxial materials.