Abstract: As a new kind of micro-pattern gas detector (MPGD), the thick gas electron multiplier (THGEM) was investigated to detect higher energy photon in this paper. In order to understand the reaction mechanism and the main factor influencing detection efficiency, three models consisting of interaction model between photon and detector, drift and diffusion model of electron, and ionization model of gas were developed based on multi-particle transport software, multi-physical coupling simulation software, and the gas ionization simulation software respectively. The electron emission probability with energy and angular distributions from the inner surface of the drift and the electrode of the THGEM were simulated, and the drift and lateral diffusion behavior in the electric field was dynamically simulated. Especially, the amount and entering-hole efficiency of primary electrons were quantitatively calculated. It is proved that increasing the drift distance and the working voltage of THGEM can significantly improve the detection efficiency of gamma ray.