1. A projection method of three-dimensional imaging, comprising the steps of: (a) providing a light source, wherein the light source is to generate a radiation field for being projected onto a detecting module, which includes a plurality of detectors arranged in an array pattern; (b) piling between the light source and the detecting module a plurality of three-dimensional sub-voxels to form a voxel in a form of 3D matrix; (c) defining a sub radiation field between each of the detectors that have detected the radiation field and the light source; (d) basing on each of the three-dimensional sub-voxels in the sub radiation field with respect to each of the detectors to perform a calculation so as to obtain a sub-geometric factor, the calculation further comprising the steps of: (d1) projecting each of the detectors, the sub radiation field respective to each of the detectors, and the plurality of three-dimensional sub-voxels onto a first detection plane and a second detection plane, respectively, so that each of the detectors, the sub radiation field respective to each of the detectors, and the plurality of three-dimensional sub-voxels form correspondingly a plane detector, a plane radiation field and a plurality of 1D arrays in parallel to further form a two-dimensional plane on the first detection plane and the second detection plane, respectively; wherein each of the 1D arrays has a plurality of grids, and wherein the first detection plane is expanded by a first axis and a second axis and the second detection plane is expanded by the first axis and a third axis; (d2) rotating the plane detector to the first axis of the first detection plane and the first axis of the second detection plane, respectively, so as to generate a varied coordinate between the plane detector and the plurality of grids; (d3) basing on the varied coordinate to calculate a corresponding geometric ratio value for each of the grids of the plane radiation field with respect to each of the plane detectors to occupy the first detection plane and the second detection plane, respectively; and (d4) multiplying the two geometric ratio values of each said grid with respect to the first detection plane and the second detection plane so as to obtain the sub-geometric factor of the three-dimensional sub-voxel with respect to each said detector; and (e) collecting all the sub-geometric factors of the three-dimensional sub-voxels with respect to the individual detectors in the sub radiation field to form a geometric factor matrix.