3D-FEM computation model included 41 items large structural planes and 5 items faults in the real attitude and distribution in the No.3 dam foundation and No.4 dam foundation of left powerhouse of the Three Gorges Project combined action are created. The safety factor of No.3 dam foundation in deep sliding stability due to No.4 dam foundation combined action is studied. A new practical judge method of dam foundation stability safe factor based on 3D-FEM simulation results using strength reduction method is proposed; it has definite physics meaning. It shows that the combining action of No.4 dam foundation can increase the safety factor of No.3 dam foundation; but its effect is small.