Organic-inorganic metal halide perovskite-based photodetectors (PDs) have attracted great attention because they exhibit extraordinary optoelectronic performances due to advantages such as a low trap-state density and large absorption coefficient. As a buffer layer, G a 2 O 3 can block electron hole recombination, passivate an Si surface, reduce trap density, and improve the ability of electron tunneling. Here, we demonstrate a trilayer hybrid structure (S i/G a 2 O 3/C H 3 N H 3 P b I 3) composed of an n-type silicon wafer, G a 2 O 3 interlayer, and C H 3 N H 3 P b I 3 thin film. The effect of different G a 2 O 3 layer thicknesses on the characteristics of a PD was studied, which shows that the responsivity first increases and then decreases with an increase in the G a 2 O 3 film thickness; the optimized G a 2 O 3 thickness is 300 nm. Additionally, the optimal responsivity, detectivity, and the rise and decay times are 7.2m A W -1, 7.448×1010 Jones, and 39 and 1.7 ms, respectively. This device has a better performance because G a 2 O 3 and perovskite have a matched energy level. We believe our work could provide a new way to fabricate high-performance optoelectronic devices.