Because of the easily construction, the lower cost, the shorter construction time and less damage to the dike，the seepage measure method in the site pressurized-water test of cut-off wall is a kind of recommended examination method. But it is not popular because of the lack of suitable calculation formula of seepage coefficient. A seepage coefficient approximate calculation formula of pressurized-water test in drilling is derived by analyses of seepage distribution characters and the saturated and non-saturated three dimensional seepage computations in this paper. The result provides a certain technical support for fast examination work of the dike cut-off wall construction quality.