This paper deals with a equivalent theory between the constructive plane dispersed by the technique of splitting node and the uneven dislocation plane, which lie in the elastic and viscoelastic medium, and judging from this the displacement field expressed with FEM dislocation formula are equivalent to that with Volterra dislocation formula. Thereby, the unified theory and method calculating the displacement field and the stress field induced by the fault plane slip are obtained. Namely, the displacement or the displacement rate on the fault plane are inversed based on Volterra dislocation formula and geodetic date, and then the displacement field and the stress field are calculated by means of FEM dislocation formula and the inversed value of the fault dislocation.