In the progress of the sea reclamation engineering,the hydraulic condition is very complicated,and has much more effects on the safety of the blocking-off engineering.Traditional method is difficult to solve this problem.Then the hydraulic calculations are conducted for the sea reclamation engineering.Based on the analysis of the hydraulic conditions,we can choose the suitable scale,the right place for the entrance,and the proper maintainment procedure of blocking-off.The results provide scientific basis on the cofferdam blocking-off.