A projection method of three-dimensional imaging includes the steps of respectively projecting a radiation field emitted from a radiation source with respect to one specific detector of a plurality of detectors and a three-dimensional sub-voxel onto two two-dimensional planes; rotating the specific detector to one specific axis of the two dimensional plane; performing a calculation for obtaining a sub-geometric factor corresponding to each specific detector and each voxel; and, finally, forming a geometric factor by combining each sub-geometric factor defined by each detector and each voxel.