Foundation pits adjacent to river face many problems such as insufficient construction space, site overloading, and large deformation of the retaining structure. Accordingly, a new type of composite retaining structure, underground diaphragm wall + multirow steel sheet piles, is proposed. Field monitoring test is conducted on Xingzhuang Road station of Phase 1 of Ningbo rail transit line 5, the deformation characteristics, effect, and cost of the different retaining structures are compared, and optimization suggestions are put forward. The results show the following. (1) The deformation curves of both sides of the diaphragm wall are almost the same, showing a concave shape, small at both ends and large in the middle, whereas the multirow steel sheet piles outside the pit show a foot shape, small at top and large at bottom. (2) Compared with the data of the south retaining structure using conventional diaphragm wall, the horizontal displacement of the north retaining structure using composite retaining structure is 7%~24% less, indicating a better support effect. (3) The horizontal displacement at the diaphragm wall bottom is relatively large, but is still within the safe range. (4) The composite retaining structure occupies a small area for cofferdam, which is conducive to protecting adjacent river. (5) The composite retaining structure also has the advantages of convenient construction, short construction period, and higher sustainable economic benefits.