For maximally preserving original information in data mapping of 1:50 000 geochemical stream sediment survey, the triangulation of MapGIS is generally used to draw flat contour map; nevertheless, because of distribution, the mapsheet edge cannot be effectively controlled, and hence the map is frequently incomplete. By using the discrete data gridding of golden software Surfer to interpolate the non-uniform distribution data of 1:50 000 geochemical stream sediment survey, and using the blank to performing extension processing of the interpolation data, we can reduce the extrapolation area of the discrete data gridding. Using the triangulation of MapGIS to draw geochemical map with the extended data, we can solve the problem of incomplete filling mapsheet in the triangulation drawing of the flat contour map, thus guaranteeing the originality, truth and completion of the geochemical map.