In order to improve the theory and technology of the flowing field method that is used to detect the entrance of dam leakage, this work started from the numerical modelling of a quasi-flowing field. The physical quantities of the quasi-flowing field for a typical dam are calculated by the finite element method and main distributing features of variables for the quasi-flowing field have been obtained. Result shows that intensities of the electric field, electric potential and current near the source had a peak distribution, and it decreased dramatically to a stable level at the far field. Due to the existence of leakage, an abnormal distribution of the current intensity and electric filed appeared near the entrance of leakage in the water area. This abnormal can help detect the leakage entrance and the detecting device requires a smaller division value and high anti-interference ability.