Spaces for backfills are often constrained and narrowed when retaining walls must be built close to existing stable walls in urban areas or near rock faces in mountainous areas. The discrete element method (DEM), using Particle Flow Code (PFC-2D) software, was employed to simulate the behavior of cohesionless soil with narrow width behind a rigid retaining wall when the wall translation moved away from the soils. The simulations focused on the failure model of the soil when the movement of the wall reaches the value where active earth pressure occurs, and the shape of the sliding surface was captured. Then, based on the limit equilibrium method with the obtained slip surfaces in PFC-2D, a simplified analytical method is presented to obtain a solution of the active earth pressure acting on rigid retaining with narrow backfill width. The point of application of the active earth pressure is also obtained. The calculated values agree well with those from physical tests in the previous literature. Furthermore, the effects of the width of the backfill, internal friction angle of soil, and wall-soil friction angle on the distribution of active earth pressure are discussed.