The soil resistance factor in the internal force analysis and deformation law of plastic concrete cut-off wall is widely obtained by local testing. That the test method has the influence of the complex factors and relies on experience makes the poor accuracy of determination value. This paper gives a method of relatively accurate numerical of soil resistance factor using related theory and field monitoring information. Depending on a reinforcement project that ferroconcrete cut-off wall is concreted on the original plastic cut-off wall; the soil resistance factor is inverted by combining the backfill soil monitoring information with method in the engineering foundation. It gives the analytic equation

influence law in the construction and effective thought about inversion

which establishes the foundation of accurately analyzing the internal force and deformation of cut-off wall.