In order to improve the air-stability of organic-inorganic lead halide perovskite, two kinds of all-inorganic perovskite CsPbBr3 nanocrystals with different sizes were synthesized by high temperature hot-injection method. Because the nanocrystal size plays a key role in determining electrical performance of nanocrystal film, the effect of CsPbBr3 nanocrystal size on the photoelectric response of thin film photoconductive photodetector was studied. The results show that the photocurrent of nanosheet film is 100 times higher than that of nanocube film, resulting from the large carrier diffusion length of 2D CsPbBr3 nanosheet film with less boundary. Finally, an overall performance evaluation on nanosheet film photodetector with higher photoreponse was conducted, and low noise equivalent power and high specific detectivity demonstrate its good weak light detection capability.