Abstract: A 1D finite element method in time domain is developed, which is used to calculate the in-plane wave motion of free field in elastic layered semispace by oblique seismic incidence. First, the layered semispace is discretized virtually according to the propagation characteristic of elastic wave. Then, the finite element method with lumped mass and the central difference method are combined together to establish the 2D wave motion equations. On the basis of the discretization principle and by the advantage of the explicit finite element method, the 2D wave motion equations can be transformed into 1D equations. By solving the 1D equations, the displacement of nodes in one vertical line can be obtained. Finally, the wave motions of the whole free field are determined on the basis of the characteristic of traveling wave. As an example, P-wave is assumed as the incident wave. Theoretical analysis and numerical results demonstrate that the proposed method possesses high accuracy and good stability.