Currently the less effort of research on the theory of space earth pressure behind retaining wall adjacent to existing basements exterior wall has been made. the existing plane strain state theories can’t satisfy the calculation requirement of the earth pressure behind retaining wall adjacent to existing basements exterior wall when the length height ratio B/H of retaining wall is smaller. In this paper, soil arching effects principle is introduced into the space earth pressure calculation model built by GU[8] and a model for calculating space earth pressure is developed considering soil arching effect and existing basements exterior wall. Based on the relationship between the failure space and the distance of two walls, the soil between the retaining wall and the existing basement exterior wall is divided into four districts, I, II, III, IV. Formulas for calculating the space active earth pressure in every district behind the retaining wall are derived by establishing horizontal and vertical static equilibrium differential equation of the horizontal differential element taken from every district. The Formulas can be applied to calculate the active earth pressure wherever behind the retaining wall. Furthermore, the method of resultant force of the space active earth pressure and its position are also put forward. The spatial distribution of active earth pressure behind the retaining wall can be seen visually through the calculation of the numerical examples. Two conclusions can be drawn as follows: When the spatial effect exists, the active earth pressure distribution considering the soil arching effect along the retaining wall is quite different from that when the retaining wall is in the plane strain state; that is, the active earth pressures behind both ends of the retaining wall are much smaller than that in the plane strain state. The position of the active earth pressure resultant force point considering spatial effect is higher than that in plane strain state; and when the B/H gets smaller，the influence of spatial effect on the distribution of active earth pressure along the retaining wall and the position of the resultant force point will increase.