By using the successive approximation method,a wave equation,in which the second order ofapproximation is taken into account,has been derived from the basic equations of hydrodynamicsin Lagrangian coordinates.The reflection and refraction of a large amplitude plane sound waveon the plane interface of two non-mixing liquids are studied.It is interesting to point out that thereflection and refraction coefficients of second approximation not only depend on parameters ofmedia,but also relate to the position of sound source.It is found that the amplitudes of the secondorder reflected wave and refracted wave do not keep constant at the wavefront anymore as inlinear acoustics.Hence there are reflected wave and refracted wave which propagate in the directionother than that of waves,travelling along the directions defined by laws of reflection and refractionin linear acoustics.The nonlinear effects have to be taken into account in order to understand thephysical meaning of the results reported by Muir et al.. By using the successive approximation method, a wave equation, in which the second order ofapproximation is taken into account, has been derived from the basic equations of hydrodynamics in Lagrangian coordinates. Reflection and refraction of a large amplitude plane sound waveon the plane interface of two non-mixing liquids are studied. It is interesting to point out that there islection and refraction coefficients of second approximation not only depend on parameters ofmedia, but also relate to the position of sound source. It is found that the amplitudes of the secondorder are reflected wave and refracted wave do not keep constant at the wavefront anymore as in linear acoustics .ence there are reflected wave and refracted wave which propagate in the directionother than that of waves, traveling along the directions defined by laws of reflection and refractionin linear acoustics. nonlinear effect have to be taken into account in order to understand the physical meaning of the results reported by Muir et al ..