The analytic solution to the coupled-mode equations for degenerate fo ur-wave mixing (DFWM) in nonlinearity optical fiber (HNLF) under pump depletion is described in detail.The expressions of output phase and power for every waves are given by the first and third kind elliptic integrals,as well as the Jacobian elliptic functions.In order to facilitate the calculation and analysis, we give the commands of the special functions in Matlab.The condition of multi -level phase regeneration based on DFWM and the influence of initial pump phase and power on phase transfer curves for phase-sensitive amplifiers (PSAs) are analyzed by using the analytic expressions.Adjusting the initial pump phase allows the phase transfer curve of the PSA to match the multi-level phase of the input signal.The phase fluctuation suppress ion performance can be optimized by appropriately choosing the fiber length or pump power,in which the optimized in itial pump power is inversely proportional to the fiber length approximately.The performance of the 8phose sh ift keing (8PSK) sign al regenerator based on PSAs is simulated by Optisystem optical simulation software,which is consistent with th e theoretical analysis.The phase fluctuation suppression ratio is up to 4.62for the normalized input phase fluct uation of 0.1,and the symbol error rate without forward error correction can be reduced by two orders of magnitude.