To date, the high frequency scattering law of seismic waves in sedimentary river valleys has not been established. In this study, we adopt the high-precision indirect boundary element method (IBEM) in a two-dimensional broadband analysis of P, SV, and Rayleigh waves. The numerical results demonstrate that the IBEM is effective and accurate in simulating seismic wave broadband scattering. In low and medium frequencies (dimensionless frequency η><5.0), the seismic wave amplification effect in valleys is significant, and it attenuates at high frequencies (η>10.0) and even exhibits a necking effect. In terms of surface displacement amplitude, with a decrease in the incident angle θ, the maximum ground displacement amplitude shows a gradual decreasing trend. The surface displacement amplitude changes more dramatically with increases in the incident frequency η. Meanwhile, the focusing effect of seismic waves in sedimentary valleys also has a significant impact. When the incident wave frequency increases, the sedimentary valley displacement amplitude variation range of the valley terrain is more intense. With an increase in the valley depth, the bandwidth in the displacement amplification frequency gradually decreases, with the frequency at first peak decreasing and the spectrum curve strongly oscillating at low frequency. With an increase in depth of the sedimentary valley, the bandwidth of the displacement amplification spectrum decreases, the frequency at the first peak is reduced, and in the low frequency band, the spectrum curve is sharp. The damping ratio has a significant impact on the surface displacement amplitude of the sedimentary valley, which clearly attenuates, especially at high frequency. To more scientifically interpret earthquake damage and to better evaluate seismic safety, seismic response analyses of actual sedimentary valleys must take several factors into consideration, including wave type, incident frequency, and angle, as well as the depth-to-width ratio of the valley.