中文摘要：

基于波陡很小的假设,利用摄动法,讨论了任意深度的二维不可压缩、无黏性、无旋的三层流体系统.在刚性上边界、平底不可渗透条件下,给出了界面内波传播的统一理论以及描述其波剖面的近似非线性演化方程（NEEs）.最后讨论了几种特殊情形下的近似NEEs.结果表明文献导出的理论结果为本文的特殊情形。

英文摘要：

Based on the small steepness parameter assumption,the three-layer incompressible,inviscid and irrotational fluid system of arbitrary depth is discussed by using the perturbation method,and a unified theory of nonlinear interfacialinternal wave propagation and the approximate nonlinear evolution equations（NEEs） for interfacial-internal elevations are given on the basis of the rigid upper boundary and the flat impermeable bottom.At last we also discuss on NEEs arising from various limiting cases of fluid depth.It is also noted that the theories obtained from the present work include the theoretical results derived by Yoshimasa Matsuno（1993） as special cases.