Abstract: Under large load, plastic deformations occur in foundation soil below Bernoulli-Euler girder. Linear algebra equations were derived after the Winkler foundation beam governing equations were discretized with Finite Difference Method. Computer program Fortran was called to solve these equations. Then by p-s curve through iterative computations, beam deflection considering subgrade reaction coefficient variation with Load was obtained with Finite Difference Method. Results indicate that by p-s curve and Winkler foundation beam model considering subgrade reaction coefficient variation with load, one can approximately judge whether supporting ground soil is in elastic stage or elastoplastic stage, and the deflection can be determined relatively properly. At last, the application scope of this method is discussed.