To realize searching synoptic situations, a method of drawing contours is introduced, which is based on grid data of regular square matrix. Firstly, controlling points are added according to plotting rule and irregular points linked with smooth curve. Secondly, to catch equivalent points, the method of scanning in three quadrants is put forward. The result is similar to the handmade, the lines are proper and smooth, the procedure responds quickly, and there are certain reference meanings for making our own software.