Abstract: From the angle of geological mapping of volcanic terrains, the paper pro- poses a method-the trend surface analysis of grain size distribution. The main steps are as follows: measure the standard grain sizes at various points and fill in them on a grid of 250×250m, perform quadratic and quintic trend surface analyses using the computer and construct two grain size contour maps. The feasibility of the method is discussed here, and evidence of the delineation of a central-type volcanic edifice is obtained.