In view of the common hexahedral voxel grid 3D geological model, we introduce a 3D geological model partitioning algorithm. In the process of cutting, the cut surface will intersect with the edge of the hexahedral element. Firstly, the geological model and the cut plane are projected into the same 2D plane at the same time, the process of portioning between geological model and cut plane in the 3D is converted to an operation that looks for the intersection point between the line and the quadrilateral grid. In order to reduce the number of judgement intersections, we would find out the possible intersection area by the Line slope direction of cutting plane, then judge whether there is an intersection carefully. Next, we find other points of intersection that could not be got by projection, and connect the node coordinates according to certain rules form the quadrilateral grid and triangulation. Finally, we display the TIN. Experimental results prove that this method is feasible.