As the photocathode practical application mode, transmission-type structure has important signitication for the research. A MOCVD epitaxial p-type GaN, which has sapphire substrate and AlN buffer layer, was designed as transmission-type material. This material was prepared into GaN photocathode by ultra-high vacuum surface cleaning technique and (Cs,O) activation technique. The UV spectral response of prepared GaN photocathode was measured. The testing results showed that the photocathode had obvious gate-shaped response in the transmission mode, and the maximum quantum efficiency was close to 15%. Comparing to the reflection-mode spectral response curve, the overall responses of transmission-mode were lower and long-wave response threshold moved towards shortwave. The experiment results were analyzed from photocathode material structure, epitaxial level and photocathode preparation techniques.