GaN UV photocathode has become a high-performance vacuum ultraviolet detectors in recent years. As the photocathode practical application mode, transmission-type multilayer structure and its optical property have important influences on photocathode photoemission performance. Ultraviolet transmission spectra of transmission-mode GaN photocathode were measured. The optical transmission model of transmission-mode GaN photocathode was built, and based on the model the functional relations of thin film thickness and optical adsorption coefficient with transmission spectral were deduced. The error of calculated GaN epitaxial thickness with respect to actual value is very small, and calculated adsorption coefficients are consistent with reported data. It was shown that material structure and optical property of transmission-mode GaN photocathode can be evaluated accurately by this ultraviolet transmission spectral method.