on the basis of influence of rainfall over the years on underground water flow in Tonglushan mine field the relationship between rainfall and drainage rate prior to curtain grouting was studied by means of spline interpolation on the computer. Function relation between rainfall and drainage rate in various rainfall periods was found. A better method for calculation of water cut-off rate and evaluation of curtain grout effect was proposed. The cut-off rate of curtain grout in Tonglushan mine was calculated using this method. The results proved it to be effective.