In view of the unclear critical tier width and random height selection of upper and lower walls in two-tiered reinforced soil retaining walls under seismic loads, which hinders their application, a FLAC3D model was built based on shaking table tests of two-tiered modular reinforced soil retaining walls. It analyzed face displacement, acceleration response, and seismic earth pressure distribution under different tier widths (10, 20, 30, 40, 50, 60 cm) and height ratios (3/7, 4/6, 1/1, 6/4, 7/3). Results revealed that facing displacement first decreased and then increased with tier width, hitting the minimum at 50 cm. The acceleration amplification factor rose nonlinearly with wall height, with attenuation in the upper wall at 40 and 60 cm widths. The lower wall's earth pressure was smallest at a 50 - cm tier width. The peak facing displacement was minimized when the height ratio was 4/6. The upper wall's maximum acceleration amplification occurred at a 6/4 ratio, while the lower wall's was at 1/1. The upper wall's seismic earth pressure was highest at 7/3, and the lower wall's at 1/1. Comprehensive analysis showed a critical tier width near 50 cm at a 1/1 ratio. At a 20 - cm tier width, upper and lower wall heights should differ moderately. These findings offer data support for the design of two-tiered reinforced soil retaining walls, aiding in tier width and height configuration.