For the deep and long foundation pit affected by confined water, based on the foundation pit of the second expressway of Guangzhou New Bai-Yun international airport, the study on the bearing characteristics of diaphragm walls under different confined water levels was carried out. First, the monitoring data of the foundation pit under low-pressure water levels were analyzed. Then, simulation results from Midas GTS/NX were compared with the monitoring data, and the numerical method was also used to analyze the effect of 3 m, 6 m, 10 m confined water level after foundation pit excavation on the horizontal displacement along wall depth, the surface settlement and the axial forces of supporting. The results show that: (1) The horizontal displacement along wall depth significantly increases, and the maximum value is located near the bottom of the foundation pit. (2) The surface settlement appears to be a "grooving" shape, which increases first and then decreases, with the increase of outer distance from the diaphragm wall. The maximum settlement appears at 0.67 times the excavation depth of the foundation pit. As the confined water level rises, surface subsidence increases significantly. (3) The axial forces of the first supporting gradually decrease, and the axial forces of second and the third supporting significantly increase with the increase of confined water level. The research results provide the basis for the design and construction of the deep and long foundation pit affected by the high confined water level in coastal areas.