In practical engineering, the design process of most retaining walls must take into account earthquake resistance. It is crucial to prevent the overturning of retaining walls, particularly when the foundation's bearing capacity is weak. In this paper, the seismic active earth pressure (ES) of a relieving retaining wall in RB displacement mode is taken as the research object. By observing shear dissipation graphs across various operating conditions, the distribution law of each sliding surface is summarized, and three typical failure modes are obtained. The corresponding calculation model was established, and the earth pressure in each region was derived by the inclined strip method combined with the limit equilibrium method. Then the resultant force and its action point were obtained. By comparing the theoretical and numerical solutions with the previous studies, the correctness of the derived formula is proved, and it is found that the formula can be applied to static conditions. The variation of earth pressure distribution and resultant force under seismic acceleration are studied. The unloading plate’s position, the wall heel’s length, and seismic acceleration will weaken the unloading effect. On the contrary, the length of the unloading plate and the friction angle of filling will strengthen the unloading effect.