In this paper, the external corner with equal lengths on both sides in the foundation pit is studied. Firstly, the failure mode of the external corner with equal lengths on both sides is analyzed, and two different failure models are established according to the different side length of the external corner. Then, according to the horizontal micro-layered limit equilibrium analysis method, the basic differential equation for the theoretical calculation of the active earth pressure with equal length on both sides is established, and the calculation formula of active earth pressure for the external corners with equal length on both sides and without considering cohesion is derived when the rotation angle of the external corner is 90°. Finally, the theoretical calculation results are compared with those of the numerical simulation software. The displacement cloud map obtained by the numerical simulation is basically consistent with the calculation model used in this paper, and the magnitude and distribution law of active earth pressure calculated in this paper are basically consistent with the three-dimensional simulation values of Midas/GTS software, which proves the feasibility and rationality of theoretical calculation.