The subway station in a coastal city is located in soft soil, the diaphragm wall is used as the enclosure structure. The trench wall is unstable in the construction process. In this paper, the causes of trench wall instability are analyzed and a series of measures are taken. The results show that the ratio of wall protective mud and the time of trench formation are the main factors affecting the stability； the extra-deep diaphragm wall, needs to comprehensive analysis from the slotting time, the weight of the steel cage and other aspects to decide the framing design； for L -shape and Z -shape and other special amplitude diaphragm wall, the use of the three -axis mixing pile to reinforcement the trench wall, can greatly improve the stability of trench wall and ensure the safety of engineering construction.