The influences of nonlinear effects on sensing performance of forward stimulated Brillouin scattering (FSBS) are investigated using opto-mechanical time-domain analysis sensor, which is taken for example. The excitation of FSBS often requires high pulse power (at Watt level) because of the lower gain coefficient. Owing to the co-propagation of reading pulse and scattered light, high-power activation pulses will induce various nonlinear effects in an FSBS sensing system. Using the reported method based on activation-reading time-domain separation, the influences of nonlinear effects due to activation pulses can be effectively avoided. However, the nonlinear effects of reading pulses directly affect the sensing performance. Based on this consideration, we study the influences of nonlinear effects on FSBS sensing and their physical mechanisms under different values of peak power of reading pulses; the variation process of the 1st- and 2nd-order FSBS spectrums along ～4.7 km standard single-mode fiber are revealed in detail. Finally, the optimal region is found, in which a perfect FSBS local spectrum is obtained, and the sensing distance can be extended.