Abstract: Based on the bilinear elasto plastic model for soil and on the deformation consistency of pile and soil, the analytical solutions of composite modulus in a multi element composite foundation under both elastic and plastic conditions are obtained by applying parametric variational principle of minimum potential energy.The results show that the solution in elastic condition is in accord with that calculated by conventional area weighted average method,while the solution under plastic condition is related to factors including load and the ratio of soil stiffness under elastic condition to that under plastic condition.At last,all the solutions are verified by ANSYS. The method provide a new way to calculate the composite modulus of multi element composite foundation.