For optical coherent M-ary phase-shift-keying (M-PSK) system, the frequency offset algorithm based on differential phase or FFT maximization which was widely used is difficult to achieve MHz estimation error when the data length is short, which is difficult for the following carrier phase estimation to recover the data. To meet the needs of high accuracy and real-time performance for frequency offset estimation in the M-PSK system, a frequency estimator based on fast Fourier transform and multiple signal classification (MUSIC) was proposed and investigated. For the first time, MUSIC algorithm was used in this area. The proposed algorithm is accurate especially when the data length is short. The principle and flowchart were proposed to illustrate the algorithm. Numerical simulations of 20-Gbaud QPSK coherent systems were carried out to demonstrate this algorithm.