Spatially modulated full-polarization imaging systems can achieve synchronous detection for full-polarization parameters of targets. The existing demodulation algorithms are only used in monochromatic light detection systems, and they are not suitable for polychromatic light detection systems. The polarization-demodulation algorithm can be modified by finding the position of the central spectrum to obtain the actual carrier frequency. The experimental results demonstrate that the polarization information of the central wavelength can be obtained by utilizing the modified image-demodulation algorithm in broadband spatially modulated full-polarization imaging systems with less than 5% polarization-detection error. The experimental results provide a guild for analyzing the demodulation algorithm of full polarization imaging systems with polychromatic light.