1. A reduction method for a boundary artifact on the tomosynthesis, comprising the steps of: performing a projection process, further including the steps of: a light source generating a radiation field within a limited angular range to a detection module; wherein the radiation field projects light beams at different predetermined angles onto the detection module so as to obtain corresponding projection data at different directions, the detection module has a plurality of sensors arranged in an array pattern, a sub radiation field is defined to be a localized radiation field between one said sensor and the light source; forming a 3D spatial array by accumulating the projection data at the different directions so as to stack a spatial array having a plurality of spatial elements, the 3D spatial array including a first dimensional array, a second dimensional array and a third dimensional array; obtaining a plurality of sub geometric factors, each of said sub geometric factors being obtained by carrying out a calculation upon each said spatial element of the sub radiation field with respect to each said sensor; and forming a geometric factor matrix by stacking the plurality of sub geometric factors with respect to the corresponding spatial elements; performing a back projection process upon the geometric factor matrix so as to obtain a back-projection geometric factor matrix; and adjusting a boundary area of the back-projection geometric factor matrix.