As a special type of site effect, the influence of basin effect on ground motion is still being studied. In this paper, a two-dimensional trapezoidal sedimentary basin model was established with the explicit finite element method. According to Buckingham's π theorem, a number of dimensionless parameters affecting the basin effect were defined, i.e., basin depth-width ratio, wave velocity ratio of medium, ratio of basin depth to incident wavelength, and ratio of basin width to incident wavelength. Using Ricker wavelet as the input wave, the influence of above-mentioned dimensionless parameters on the amplification effect of the basin model was studied under vertical incidence of plane SH. It was found that the basin-induced focusing effect becomes obvious with increasing depth-width ratio of basin, and the basin edge effect becomes obvious with decreasing depth-width ratio of basin. With a certain depth-width ratio of basin and incident wave ratio, the maximum amplification coefficient of the basin gradually moves from the center to the edge with increasing ratio of basin width to incident wavelength, indicating more and more obvious edge amplification effect. The wave velocity ratio difference has the greatest effect on the amplification coefficient. With the increase of wave velocity ratio difference, the wave impedance ratio increases and the amplification effect becomes obvious. This study makes the research conclusion on the amplification effect of ground motion in trapezoidal basins have more general significance.