Integrated three-phase AC-DC system is widely used in modern society with series of base units. In this paper we deduce a general equation of the system in an affine coordination in order to use the differential geometry. Firstly, a new approximate equivalence method is presented by differential homology, including the judgment criterion and the main parameters of the equivalent model. Besides, the chaotic analysis relates different parameters to different situations which can accurately be simulated by PSCAD/EMTDC. And then calculation of eigenvalue indicates that the equivalence method is useful and simple. Finally, a simulation shows the accuracy and the applicability of the method.