Characteristic information of FBG reflection spectra changes because of the existence of demodulation system error in the actual demodulation. The analysis of spectra for FBG under axial uniform force based on particle swarm optimization is given in this paper. Objective function is established and force experiments and numerical simulation of fiber Bragg grating have been made. The characteristic information of FBG reflection spectra under axial uniform force is extracted and theoretically reconstructed. The results show that, the improved particle swarm algorithm in fiber grating reflection spectra analysis is feasible. Also, the method has other characteristics such as high accuracy, good stability and fast convergence speed.