Nonlinear phase noise structure is studied in the background of long-haul interferometric fiber sensing system. The influence of each factor is analyzed and evaluated in detail, and suppression methods are discussed briefly. The results show that phase noise of the system mainly includes the phase noise converted from intensity noise, the phase noise caused by nonlinear effects induced laser linewidth variations as well as the phase noise introduced by self phase modulation and cross phase modulation. Stimulated Brillouin scattering and four-wave mixing can cause intensity noise and then it turns into phase noise. For the fiber sensing system which has a narrow detecting bandwidth, the corresponding noise induced by four-wave mixing can always be neglected. Stimulated Brillouin scattering, four-wave mixing and modulation instability can cause laser linewidth broadening, leading to the increase of phase noise. When the number of channels is large, the phase noise introduced by cross phase modulation cannot be ignored. The obtained results provide a good guidance for the practical applications of long-haul interferometric fiber sensing systems.