For avoiding the problems caused by solving large linear equation systems numerically, the authors introduce the basic concept of the electrical reflectivity tensor and the basic idea of the quasi-analytical approximation into D. C.electrical potential field, and define three auxiliary functions. Simple comparison shows that the surface charge density reflection function is a function of the free surface charge density on the surface of the anomalous body under consideration. In simple situations described above, the function reduces into a simple proportional relationship. The same is true for the surface potential reflection function in simple cases.