Abstract: By use of the WKBZ approximations of eigenfunctions with a surface phase-shift correction, and taking account of both the waveguide and bottom-reflection modes, a WKBZ adiabatic mode approach suitable for gradually range-dependent channels is proposed. Numerical simulations show that the acoustic fields in convergence zones are mainly determined by waveguide modes while for deep shadow zones bottom——reflection modes must be taken into account. This approach is applied to the environment in the Phillipine Sea, and the calculated results for frequencies from 109 to 860 Hi and ranges up to 250 km are in good agreement with the measured data.