Abstract: In order to solve the problem that the prediction of the buoyancy of underground structures in confined aquifer is inaccurate, a series of model tests on the impact of confined aquifer seepage on the buoyancy of underground structures are conducted. Two cases are studied: (1) the vertical seepage in homogeneous soil layer; (2) the vertical seepage of the confined water through overlying aquiclude. The influences of the seepage on the buoyancy of the underground structure under different hydraulic gradients in both cases are investigated. The results show that the pore pressure distribution and the buoyancy of the underground structures are not consistent with those obtained from hydrostatic pressure when the seepage occurs. And they are usually larger than those obtained from hydrostatic pressure. The raising ratio of the pore pressure and buoyancy to the theoretical value based on hydrostatic pressure equals to the vertical hydraulic gradient in case the vertical seepage occurs in homogeneous soil. The raising ratio of the pore pressure and buoyancy to the theoretical value based on hydrostatic pressure is much larger than the hydraulic gradient in case the confined water seeps in the overlying aquiclude, which is obviously different from that in homogenous soil. In the case herein, the raising ratio is about 2 times the hydraulic gradient.