Taihu Lake is located in the central area of Taihu Basin, which controls the flood situation of Taihu Basin. According to data (1998-2002) from of tour gauging by the hydrometry departments around Taihu Lake, this paper calculated the water balance in Taihu Lake, and analyzed the error. From the results, it discussed the feasibility of calculating the water balance in river plain with tour hydrometric gauging method, and suggested how to improve the precision of tour hydrometric gauging .