Foundation pit dewatering is the key to the whole foundation pit safety. Although the waterproof curtain was built in the foundation pit excavation of Nanshanhu complex building in Nantong, it is still necessary to make a safety assessment on the dewatering scheme of the foundation pit. 3-dimensional seepage model is established by using the MIDAS/GTS numerical analysis software, by the simulating analyzing a single well pumping test in the field and synthesizing the permeability coefficients of indoor tests in the geological exploration report and pumping tests, the seepage boundary function and the permeability coefficients fitted for the practical engineering situation are inversely deduced; then the 3-dimensional seepage model is established by using the parameters reversely derived to simulate the changing relation between water level and time in multi wells pumping. The dewatering effect of multi wells is analyzed to verify whether the dewatering design is reasonable in order to guide the foundation pit excavation.