The analytic method is used to analyze the range of quasi-phase matching and the evolution of the phase matching in the case of pump depletion, and the expression for the phase matching is given. The evolution process of four-wave mixing gain under pump depletion is compared to the case in small-signal approximation. In the case of pump depletion, with the increase of the transfer power, the linear mismatch factor corresponding to the total phase matching gradually tends to 0 and the quasi-phase matching range also becomes narrower. By determining the quasi-phase matching rang, the cascaded four-wave mixing process in wavelength division multiplex or space division multiplexing systems can be simplified for analysis. The small signal approximation model is only suitable for the case that the transfer power is far less than the pump power, and the linear mismatch factor at the total phase matching is not dependent on the length of the fiber.