The land subsidence directly threatens the safety of urban flood defence by decreasing the land altitude loss. According to the monitoring results of the land subsidence and the yearly highest tide height of the Huangpu River and all previous heightening and subsidence conditions of the flood defence wall in the Bund, it is shown that the amount of land subsidence in Shanghai was 1.973 m from 1921 to 2007 and the land altitude of center town is about 3.5m at present, and the land altitude of center town almost lower the highest tide height of Huangpu River and brings about the long-term flood defence pressure. The yearly average value of highest tide height of the Huangpu River is about 4.59 m, and the highest tide height gradually raises. The conditions of flood and inundation are intrinsicly link with the development of land subsidence. The land subsidence plays a important role in the four times of heightening of the flood defence wall in the Bund, and the land subsidence amount occupied 98.0%,62.6%,30.1%,7.8% respectively of the heightening of the flood defence wall. Now the existing flood defence installations have been endangered by the land subsidence, the regional land subsidence amount from 1994 to 2006 was 71.9 % of total subsidence of the flood defence wall in the Bund. The land subsidence is an importment factor of endangering the urban flood prevention safety and a key link of urban calamity system, and to give rise to our highly precautions for its long-term harmful effects. The prevention and control of the land subsidence have to be the crucial contents of mitigating and preventing natural calamities.