It occurs occasionally that the excavating pits are adjacent to as -built constructions. The soil mass located between the foundation of the as-built construction and the pit is viewed to be finite width soil mass, resulting in failure of classical finite soil mass theory. Meanwhile, the seepage also has non-negligible influence on excavation of foundation pit. In response, soil-pressure model box is designed to investigate the influence of steady seepage on active soil pressure of finite width soil mass. The corresponding numerical simulations are implemented to verify the experimental observations for a further systematic study. The following conclusions are obtained: The time-history of soil pressure can be divided into three phases, Including steady seepage, movement of retaining wall and the rest of retaining wall; The active soil pressure under the condition of steady seepage is larger than that of static water, and in the numerical simulation, the active soil pressure of the soil with finite width is smaller than that of the soil with half-infinite width; For sandy soil, the calculation result of effective stress method is close to the test result, but it is safer to compute active soil pressure by soil and water partition method in practical projects.