Based on the data observed by KOUN radar of USA on the two convective rainfall events on May 10th and June 12th in 2003, a method of estimating liquid precipitation is developed by eliminating the hail echo from hybridtype precipitation echo. This method ensures hail has no influence on estimating rainfall rate. By analysis of the differences of the four methods on estimating rain intensity at S band of dual linear polarization Doppler radar, the influence of K_DPnumerical method on the estimating rainfall is also studied here. The conclusion is that there is great difference in the four estimation methods and the K_DP method is not fit to estimate precipitation when the rain intensity is weak.