Phase noise is considered as one of the main sources of signal impairments in optical phase-shift-keying (PSK) systems. The main sources of linear phase noise and nonlinear phase noise are introduced. The linear phase noises arise from amplified spontaneous emission (ASE) of each in-line amplifier and the nonlinear phase noises are induced by the fiber Kerr effect. The probability density functions (PDF) of linear phase noise and nonlinear phase noise are obtained respectively by theoretical analysis. The probability distribution of linear phase noise, nonlinear phase noise and total phase noise in fiber links are also obtained by statistical simulation, and are compared with Gaussian distribution. The results show that the Gaussian model can reflect noise distribution in practical fiber links when input signal power is low, while failed to accurately characterize systems when the nonlinear contribution to phase noise becomes dominant.