According to the correlation between the annual water level and precipitation in the Wuxi Reach of the Beijing - Hangzhou Grand Canal and the condition of the urban water structures, the law of the water level change in the reach was analyzed in this paper. The mechanism the water level changes before and after 2003 was studied, so as to get the water level change characteristics under the condition of the water structures and climate, and summarize the effect of the urban water structures on the flow direction.