The reliability of convective cloud seeding effectiveness evaluation result is poor when using traditional statistical test method because of larger spatial and temporal variation. The floating target area method, a new method proposed by Woodley, is introduced, which is based on radar echoes. Some modifications are made in this scheme in combination with Tianjin geography and weather modification operation characteristics. The radar base data are used to determine the floating target cell recognition and tracking methods, then determine the catalytic units and control units in the floating target cells; the physical parameters and the retrieved precipitation of each unit are recorded, and the precipitation enhancement effect is calculated to provide physical evidences of artificial rainfall. The method is used to evaluate the effectiveness of convective cloud seeding on 24 July 2011 in Tianjin, for example, and the objective evaluation results are obtained, which shows that: the value of the maximum echo intensity and echo top height increased as the catalyst got into the clouds. It is proved that artificial catalysis takes effect in precipitation enhancement. The effectiveness of precipitation enhancement is 7.69% at a significance of 0.043.