The foundation pit support and cofferdam engineering of the Qingyuan No.2 ship lock project are complicated in technology. The foundation pit has a large excavation depth, a high water head, and left and right banks with different heights. The engineering geological condition is unsatisfactory, for the soil layers, such as sand, soft soil, and caves, are poor. The pit enclosure structure is only 1m away from the first-line ship lock that has been built. The foundation pit retaining structure must also serve as a cofferdam, and at the same time meet flood protection functions, structural safety, and seepage safety. A double-row piles (walls) supporting structure combined with door frame or buttress retaining wall structure are used in the engineering. Double-row piles (walls) are used to resist the water and earth pressure outside the foundation pit. The retaining wall structure meets the water retaining requirements of the cofferdam during the flood period. As a connecting plate of double-row piles, the bottom plate of retaining wall structure combines the type of the building foundation pit with the structure of the water conservancy bank. It also serves as waterproof cofferdam in water conservancy building while meeting the excavation requirements of the foundation pit. It is a new combined support structure. We take the second-line ship lock project of Qingyuan Water Conservancy Project as the background. Using the finite element method and considering the construction process, this new type of cofferdam and foundation pit support was calculated, and optimization suggestions were put forward. The actual monitoring results verify the rationality of the cofferdam foundation pit calculation and the feasibility of the scheme, which provides a good reference for the design and construction of complex water conservancy cofferdam foundation pit engineering.