In order to simplifying the analytic method of bending problem of rectangular thick plate, the decoupling method and the modified Navier method are combined for accurate bending analysis of rectangular thick plates with all edges clamped supported. By using the transition function, the basic governing equations for Mindlin plates are first decoupled into independent differential equations which can be solved separately. With the different series corns unified by modified navier method, analytic solution of rectangular thick plate with all edges clamped supported is derived simply. Numerical comparisons show the correctness and accuracy of the results at last. The method used in this paper leaves out the complicated derivation for calculating coefficients and obtain the solution to problems directly.