Abstract: The algorithm for decomposing a concave polygon into convex polygons is widely used in processing computer graphics geometry. A new fast algorithm of convex decomposition is proposed after the existing convex decomposition algorithms analyzed and induced. The fast algorithm of convex decomposition is based on adjacent angle bisector of concave point and vertex of polygon. The algorithm does not increase new vertex, and the number of convex polygons decomposed is small, size and shape is excellent. The algorithm is used in earthwork calculation of grid. The algorithm can quickly find out demarcation points of fill and dig, confirm the areas of fill and dig. Compared with other convex decomposition algorithm, the conclusions are as follows：With the decomposition way of dividing into triangles, the precision of earthwork calculation equivalents to the triangulate, but the ability of resistance gross error is stronger, computing speed is faster than the latter. With the decomposition way of increasing vertex, the precision of earthwork calculation and the computing speed is better than the latter.