The traditional 16QAM phase estimation algorithm for coherent optical systems chiefly employs QPSK partitioning.However

owing to the influence of amplifier ASE noise

the radial movement of the constellation point seriously affects the accuracy of the phase recovery.In view of the different extent of the effects

this paper makes a detailed analysis of the classified amplitude threshold in the QPSK partitioning and proposes an analytical model that takes the relationship of Optical SNR（OSNR）

the length of summing window

and the threshold radius into account.The model also induces a group of weighted coefficients into phase estimation.Monte Carlo simulation results show that the improved method effectively increases the precision of phase recovery with still lower BER.