1. A method of reconstructing a PET image, comprising: obtaining a first diffusion distribution and a second diffusion distribution, wherein the first diffusion distribution represents a diffusion degree of projection data along a direction vertical to an incident direction indicating a photon perpendicularly hitting onto a front surface of a crystal, and the second diffusion distribution represents a diffusion degree of projection data along an incident direction indicating a photon perpendicularly hitting onto a side surface of a crystal; determining position information of a first crystal and a second crystal in a projection direction; obtaining a first diffusion expression according to the first diffusion distribution, the second diffusion distribution, a point source position where a photon is generated and the position information of the first crystal, wherein the first diffusion expression represents a diffusion degree of projection data from a first incident point to the first crystal; obtaining a second diffusion expression according to the first diffusion distribution, the second diffusion distribution, the point source position and the position information of the second crystal, wherein the second diffusion expression represents a diffusion degree of projection data from a second incident point to the second crystal, and the first incident point and the second incident point are respectively two incident points corresponding to the point source position in the projection direction; obtaining a diffusion relationship according to the first diffusion expression and the second diffusion expression, wherein the diffusion relationship represents a correspondence relationship between the point source position and projection data in the projection direction; and reconstructing a PET image according to the diffusion relationship.