The excavation of adjacent foundation pit and the change of site seepage conditions will affect the earth pressure on the supporting structure of the foundation pit constructed previously, which in turn causes structural deformation. However, there are few earth pressure calculation models which consider the above two aspects at the same time. Based on the limit equilibrium method and the geometric relationship between the adjacent excavation boundary and the logarithmic spiral sliding surface, the calculation theory of the effective weight of soil and the change of suction with the seepage condition of the site is introduced to establish a calculation model of active earth pressure considering the adjacent pit excavation and seepage condition. Then the model is verified to be logical and reliable by the monitoring data. Parameter analysis shows that when the adjacent pit depth is constant, the active earth pressure coefficient first changes linearly with the increase of pit spacing, then increases and decreases, and finally stabilizes. When the pit spacing is constant, with the increase of the adjacent pit depth the active earth pressure coefficient presents two types of changes, one is the “hillside type”, which rises first and then declines; the other remains unchanged after rising. The rainfall seepage and the rise of groundwater level will reduce the suction stress in soil and lead to the increase of active earth pressure. The research results can provide a theoretical reference for the stress and deformation analysis of the foundation pit affected by the adjacent pit excavation.