We have developed an overset-grid algorithm to simplify the difficulty of curvilinear grid (CG) generation and increase the computational efficiency for seismic wavefield modeling by using the finite-difference method in areas with complex surface topography. The overset grid comprises a Cartesian grid block and an approximately orthogonal CG block. The Cartesian grid covers most of the simulation domain, whereas the CG discretizes the near-surface topography. The Cartesian grid and the CG overlap each other arbitrarily. We use sixth-order explicit Lagrangian interpolation to exchange data between the Cartesian grid and the CG, which is shown to be sufficiently accurate. We also find that spatially smoothing the source term is important for reducing strong artificial reflections when the source is near the overlapping zone. Finally, numerical tests are performed to verify that the proposed overset grid is well suited for effective numerical simulation of seismic wave propagation.