Abstract: In this paper,Zhang and Tindle's concept of the complex effective depth is cited,and the approximate equation of Chapman's (real) effective depth for Pekeris guide is extended to the complex effective depth approximation whose real and imaginary part explicates the phase change and energy loss on reflection.It is shown that the homogeneous acoustic field,which comprises the complex effective depth approximation,closely reproduces the character of low modes at small grazing angle,and calculates effectively the acoustic field at longer ranges in shallow water.Application of the complex effective depth approximation can be extended to bottoms having two soft solid layers.