A contour tree has been found many important applications in extraction of terrain structure lines, terrain analysis, etc. A regular contour tree expressing the spatial relationship of contours is difficult to build, even with data of high quality. A new method is proposed in this paper： classify contours into close group and open group; build contour tree for each group respectively; finally amalgamate these two contour trees into one. With the elevation values, spatial relationship of contours is of easy accessibility. The result shows that the above mentioned method is correct and effective.