To investigate the distribution of spatial active earth pressure on the walls of square shafts, a failure model of the sliding body in surrounding soil under different aspect ratios was established. A formula for calculating the spatial active earth pressure was derived, and the influence of physical and mechanical parameters of the retaining wall and surrounding soil on the spatial active earth pressure was analyzed. The results show that when the aspect ratio of the square shaft wall is small, the sliding body consists of an upper triangular prism and a lower wedge. When the aspect ratio is large, the sliding body is a single wedge with a trapezoidal cross-section. As the aspect ratio increases, the spatial active earth pressure at the same burial depth gradually increases. With the increase of burial depth, the earth pressure gradually transitions from a nonlinear variation to a stable state, approaching the Coulomb’s earth pressure. The critical aspect ratio of the retaining wall increases with the increase of the wall’s aspect ratio and decreases with the increase of the soil’s internal friction angle. When the aspect ratio of the retaining wall is less than 10, the difference between the spatial active earth pressure and Coulomb’s earth pressure is significant. When the aspect ratio exceeds 10, the difference is less than 10%.