Pump depletion was not taken into account in the fourwavemixingbased measurement technique for fiber nonlinear coefficient, which made the results inaccurate. To solve this, the elliptical equation with pump depletion and fiber loss was derived based on the theory of degenerate fourwave mixing. The nonlinear coefficient of the tested fiber could be obtained by setting the parameters in numerical calculation and utilizing simple mathematical method. The necessity of considering pump depletion and fiber loss was analyzed. The numerical results were consistent with the Optisystem simulation data. The improved method proposed in the can be applied to the small signal case, but also to the process of parametric saturation. This technique provided an important parameter for further designing the fiberbased 2R regenerators.