For negative-index materials(NIM) of second-harmonic wave(SHW) and positive-index materials of fundamental frequency wave(FFW)，the equations of the coupled wave was deduced，taking account of absorption coefficient，walk-off effect，phase mismatching capacity and the approximation of slow-varying amplitudes of the plane wave. On the basis of the equations，a formulation of SHW conversion efficiency was obtained in condition of small signal approximation for FFW. Through numerical simulation, the effects of phase mismatching capacity，walk-off angle and absorption coefficient of SHW on the conversion efficiency of SHW were scrutinized respectively. The maximum conversion efficiency of SHW was realized via increasing the length of NIM，that is，there exists an optimum length of NIM; however, the conversion efficiency of SHW shows an evidencet periodic evolution and the maximum conversion efficiency always diminishes. The conversion efficiency decreases with the increases of the phase mismatching capacity and the absorption coefficient. The influence of walk-off effect is not evident so that it may be neglected with respect to the conversion efficiency of SHW. This study provides a basis for investigating nonlinear theory of negative-index materials.