The soil pressure characteristics of gravity retaining walls under seismic action are an important consideration in the retaining wall design. In the seismic design codes of many countries, including China, the seismic soil pressure of a retaining wall is calculated using the Mononobe-Okabe formula. Previous studies have identified many limitations with respect to some of the assumptions of the Mononobe-Okabe formula, which has led to results that are inconsistent with actual situations. As such, the reliability of this method must be further examined. In their detailed studies of the soil pressure characteristics of retaining walls, researchers have improved the pseudo static method, developed the pseudo dynamic method, and introduced model experimentation and numerical analysis, which have greatly improved the accuracy of the calculated results. However, most research has considered maximum earthquake intensity, but not the impact of the time-history variation of seismic waves on the soil pressure behind the wall. In this study, we used numerical simulation to establish a series of monitoring points on the back of the retaining wall and then obtained time-history curves of the acceleration and soil pressure strength. Based on these time-history curves, we then analyzed the distribution characteristics of soil pressure strength, calculated the total soil pressures, and obtained the overturning moment of the toe of the wall. Finally, we compared the soil pressure distribution, total soil pressures, and overturning moment of the toe of the wall with those calculated by existing methods and codes. The results show that the peak acceleration of each monitoring point occurs at the same moment, but the peak soil pressure does not. Some existing methods do not consider the time-history change of peak soil pressure strength, thereby yielding a bigger result than is practical. In low seismic intensity conditions, the total soil pressures and overturning moments calculated by codes tend to be conservative, whereas in high intensity conditions they tend to be dangerous.