The limitation of peak power caused by two kind of nonlinear effect, self-phase modulation and stimulated Brillouin scattering, in G.652 single-mode fiber is studied. It shows that dispersion and self-phase modulation lead to distortion of optical pulse waveform and its effect is determined by optical pulse peak power.Stimulated Brillouin scattering results in fast decay of pulse power in fiber and further shorter sensing distance, and its effect is determined by optical pulse energy. Experiment data shows that self-phase modulation puts more stringent limit on optical pulse lauch power. Based on the analysis of experimental data, empirical formulas for power upper limit are given, which can be used to estimate the maximum optical pulse incident power in distributed fiber sensing systems. For a typical distributed fiber sensing system with range of 25 km, the incident power upper limit of optical pulse peak is about 1 W.