The intrusion of debris flow into a river is a common phenomenon that usually occurs in mountainous areas. Debris flow, unlike a landslide, has a complex component that makes its simulation more difficult. In this paper, a two-layer, 2D model is presented for use in the preliminary study of debris flow intruding into a river. In this model, the upper layer is assumed to be a single-phase flow and the lower layer to be a solid–liquid two-phase flow. The drag force between the fluid and solid phases of the debris flow is considered in this model. The basal friction term of the debris flow is considered as the phase-averaged value of the solid shear stresses and fluid shear stresses. A numerical method based on the finite-volume method is proposed to solve the complex model equations involved. Several numerical tests were performed to confirm the feasibility of the proposed numerical method and model. The numerical results yielded a reasonable prediction of debris flow motion and tsunami wave generation and illustrate the complex interplay between the debris flow and tsunami waves. The characteristics of the river have extra effects on the distribution of subaqueous debris flow (e.g. flow velocity) and should not be neglected.