Nonlinear process of the acoustic pulse propagation in a planar wave-guiding channel is considered.The hydrodynamic set of equations is reduced to the nonlinear wave equation. An asymptotic approach is proposed for solving the nonlinear wave equation, enabling, without additional restrictions, to separate the problem on determination of the wave field transverse distribution and on analysis of the nonlinear dynamics in longitudinal direction. Analytical description of the pulse mode structure and nonlinear dynamics of the envelope are presented with account of longitudinal inhomogeneity of the wave-guiding channel.