Abstract: For finding the ultimate bearing capacity of a foundation with given size,we can solve this problem starting with dividing the foundation soil.Then suppose a uniform load act on the foundation,using calculus answer of Boussinesq’s formula and corner-points method to get extra stress of every mesh knots by MATLAB program,then,find the principal stress.Combining with failure criterion,use MATLAB program to find coordinate of points witch were destroyed.Using the graph processing function of MATLAB,we can found out the failure points in the coordinate.Enlarge the uniform load and find the failure point until these points can form a failure surface.This uniform load is the ultimate bearing capacity of the foundation.The method can avoid errors bring by unreasonable hypothesizes of the experience forums,and we can get the three-dimensional failure surface of the foundation soil.