In order to control the excation risks of deep foundation pits in subway stations, it is crucial to identify and address quality defects in the construction of diaphragm walls. This paper takes the deep foundation pit project of a subway station in the central city of Nanjing as the background, aiming at the situation that quality defects are easy to occur in the construction of underground continuous wall, which leads to the risk of deep foundation pit project. Before the excavation of foundation pit, the construction steps that are easy to cause quality problems such as long trouting time of underground diaphragm pit, failure of section steel joint and failure of continuous concrete pouring are sorted out and analyzed, and the core-taking and some-penetration detection measures are adopted to further verify the ground connecting wall with stress and water sealing defects. Combined with the defects of underground continuous wall sorted out above, pre-control countermeasures such as pile filling, reinforcement reinforcement and joint water sealing are adopted. In addition, in the process of foundation pit excavation, the defective joint is timely closed with steel plate along with excavation, and the leakage reinforcement of underground continuous wall is repaired. Finally, the construction of deep foundation pit of the subway station is successfully completed, which can provide reference for the combing and pre-control of construction defects of similar deep foundation pit.