Abstract: This paper deals with the intersection principle of a D.S.type analytical plotter by using three planes to determine the model point.Of all these planes,one passes through the model point while the other two planes are the so called "x-plane" and the "y-plane",passing through one or two corresponding image points.After that,the unknown image coordinates to be controlled by a servo-system are calculated by a collinear epuation the same as that on an AP type analytical plotter.Based upon this principle,the author has derived several formulae for the different types of operation(point determination,contouring and profiling) on a D.S.type analytical plotter.