We investigate the influences of the phase noise induced by the demodulation module on the performance of subcarrier M-ary phase-shift keying (MPSK) optical communication system over turbulence channel. Novel asymptotic expression of average symbol error probability (SEP) is derived based on the Meijer’s G function and Fourier series over Malaga turbulence. The Malaga channel model used in our study can be potentially applied to the performance analysis problems involving other turbulence channels. The influences of the phase noise and its truncation error on the SEP performance are analyzed under different turbulence conditions, phase noise deviation, and modulation order. It is illustrated that the phase noise significantly degrades the performance of the system over turbulence channel. The more truncated number of Fourier series is required to get the minimum truncation error when the phase noise is lower. Under turbulence conditions, the appearance of symbol error rate floor depends predominantly on the phase noise.