Hongdu Bridge is an extraordinarily huge bride across northern branch Ganjiang River, in the city of Nanchang. Main tower adopted single column slanted pylon space twist face back cable structure. Its construction is based on double-wall cofferdam method. The paper discuss thecalculation of key step and key position for the key technology in terms of double-wall cofferdam and platform underwater construction, Which include the design of the base construction and double-wall cofferdam,steel cofferdam construction, the calculation and checking of the cofferdam and so on. The whole length of 9.5 km, except the starting period of about 2.4 km for roadbed type, the other sections of approximately 7 km. The project always invests about 18 Billion yuan.