Abstract: In view of the shortcomings of traditional block theory and the assumption of infinite extension of structural plane, a new determining method of structure intersection relationship is presented. On the basis of the finite extension of the structural plane, we conduct analysis by the model of structural plane disc. In calculation of the intersection of two structural planes, the difficulties of direct intersection calculation of line and circle are avoided. It chooses projection method. First, the normal plane of intersection vector of the two structural planes is obtained, and according to facts that the projection of the normal line and circle region in the normal plane are a point and a line segment respectively, so the distance between the pedal and the projection of two circle center is calculated and compared with the radius of the circular region, so we can analyze the inclusion relationship between the intersection line and the circular structure, and then judge the intersection of the structure planes. Using this principle, the selection method of in-situ structure planes in block analysis is put forward. By the selection of discriminant matrix, we ignore those short extended planes, not distinguished by traditional block theory, that do not intersect with other structural planes, or those however intersect with other planes but do nothing contribution to the formation of the blocks. This new algorithm simplifies the workload of further block calculation, greatly improves the efficiency of block analysis with large quantities of structural planes.