The foundation pit for the dry dock of Honggu immersed tunnel in Nanchang is 16 m deep and is only 15 m away from Ganjiang river at the closest position. The foundation pit is located in sand strata with 120 m/d permeability and 3~13 m water level fluctuation. The construction of the antiseepage system of the foundation pit is on the critical path of the project and the quality of the antiseepage system has great impact on the excavation of the foundation pit and on the prefabrication, transportation and immersing of the tunnel tubes. In the paper, the construction technologies for the antiseepage walls, including the reinforcement of the slot wall, the mixing proportion of the plastic concrete, the slot cutting by “two rotary drilling machines and two hydraulic grabbing machines” mode, the slot detection by ultrasonic waves, the antiseepage wall inspection by highdensity electric method, the performance of the bentonite slurry and the concrete casting, are studied. These technologies have been applied in the construction of the abovementioned antiseepage walls and good effects have been achieved. 