Abstract: The landslide-tsunami is a typical multi-hazard coupled system, characterized by complex effects resulting from the transmedia transformation of hazards. This paper proposes a two-phase Riemann-SPH model for landslide-tsunami simulation that incorporates dynamic seepage and is validated against laboratory experiments. The incorporation of dynamic seepage effects enhances the completeness of the momentum exchange mechanism in the granular landslide-tsunami process, reducing the errors in the maximum wave amplitude (a_m) and maximum wave height (H_m) by at least 24.72% and 41.95%, respectively. The results reveal a synergistic regulation of tsunami characteristics by the sliding surface inclination (α) and the landslide leading edge inclination (β): as α increases, the a_m and H_m exhibit a single-peaked, nonlinear increase-then-decrease trend. The influence of β shows a distinct piecewise pattern: when α+β < 90°, both a_m and H_m increase significantly with the angle. Beyond this threshold, non-monotonic variations appear, reflecting a competition between the increasing landslide volume and the decreasing effective impact area. Moreover, increasing α enhances seepage, turbulent and frictional dissipation effects, accelerating energy decay. These findings provide scientific support for the mitigation of landslide-tsunami hazards.