In this paper, a few kinds of non-reflecting boundary conditions are introduced, which have been used for a variety of wave propagation problems, such as acoustic waves, isotropic and anisotropic elastic waves, and waves in fluid saturated porous media. The stability of finite element methods for the acoustic wave equation with non-reflecting boundary conditions is discussed and the corresponding stability conditions are given. Some numerical results are presented to show the correctness of the theoretical analysis and the effectiveness of the boundary conditions suggested by the author of this paper and her cooperator.