The invention belongs to the technical field of three-dimensional geological modeling and particularly relates to a geological interface model construction method based on trajectory extension. The method includes the following steps of S1 calculating the external normal vector (as shown in the description) of the natural surface or excavation surface of the exposed part of the geological interface; S2 specifying the expected geological model construction direction (as shown in the description) in the horizontal plane; S3 calculating the expected extension vector (as shown in the description) of the geological interface; and S4 constructing a geological interface model of the specified extension length. The geological interface model construction method determines the extension vector of the geological interface by means of vector calculation according to the geological interface occurrence and the direction of the free face; and the geological interface model meeting the specific requirements of three-dimensional modeling and engineering analysis can be conveniently constructed according to the exposed trajectory line and the extension vector of the geological interface on the free face.