This study uses the extensively applied particle swarm algorithm to investigate the influence of the key parameters on the demodulation error using numerical simulation. The simulation results illustrate that the available range of key parameters reduces with the increase of the multiplexing number of the fiber Bragg grating (FBG) and the overlapping degree of spectral shape. Furthermore, with the maximum demodulation error as an evaluation index, we obtain a reasonable setting range of key parameters using a quantitative analysis. Under the optimal conditions, the performance of the spectral shape multiplexing demodulation of the FBG is improved, which is confirmed by the corresponding experiment.