The present invention provides a projection method of 3D ray tracing, so as to construct a geometric factor required for the reconstruction of a medical image in the imaging field. The present invention mainly projects a radiation field and a voxel detected by each detector on two 2D planes, then calculates the geometric ratio of a rectangular unit, corresponding to each voxel on the two 2D planes, to the radiation field, respectively, and multiplies the geometric ratios to obtain a sub-geometric factor with respect to the voxel. The sub-geometric factors gij of all voxels i corresponding to all detectors j are combined to form a geometric factor matrix Gij.