Abstract: In this atricle, the author studies the appliance of perspective affine homology in orthographic projection, i.e., how to evaluate the point of intersection of a straight line and a plane surface, the distance between a point and a straight line, the line of intersection of two plane surfaces, and the line of section of a plane surface and the stereoscopic etc, so as to reduce the complexity and error in drawing, increase speed of problem solving and deal with some specific problems perfectly.