Flux distribution calculation method and system when fan-shaped X-beam passes through medium

The flux distribution calculation method when the fan-shaped X beam passes through the medium comprises the following steps: measuring the energy distribution of the X ray on the detector in the absence of the medium; taking the flux distribution after the X-ray passes through the living body as projection data, and obtaining the absorption coefficient distribution of the medium through a back projection algorithm according to the projection data; discretizing a ray beam and a medium, and determining an initial coordinate of the discretized ray beam according to the position of a ray source and the position of a detector; a voxel coordinate where the ray beam is located is judged, and whether energy of the ray beam is accumulated into the voxel is judged according to whether the ray beam is in the voxel; calculating the energy of the ray beam passing through the voxel; calculating the coordinate of the ray beam after passing through the voxel; traversing the ray beam; flux in voxels is calculated according to