Abstract: Based on profound study of various position relations between a line segment and a circle,a simple and rapid clipping algorithm for circular window is presented in this paper.The basic principle is to judge the position relation between a line segment and a circular window by means of the distance from the center of the circle to the line on which the line segment lies,and the perpendicular ray of the line segment from the circle center,and then to obtain the intersecting point by rotating vector when the intersection actually happens.