This article is based on a multiband image fusion system and studies the alignment technology of the parallelism of the optical axis of the system. The five-axis parallel system includes a white light module, low light level module, short wave infrared module, long wave infrared module, and laser ranging module. The lowest light level module with the highest accuracy was 32.09. A parallelism deviation of less than 32.09 does not impact the system's usability. During installation and adjustment, the optical axis was aligned with the center of the collimator cross-target plate. This alignment produced an image size that is 99.89% of the maximum possible, which does not hinder the acquisition of image information. Finally, the system is verified experimentally using the developed platform. Experiments proved that the maximum deviation of the parallelism was nine, which is less than the maximum allowable error of the system. Therefore, this assembly and adjustment method had a certain reference value for the assembly and adjustment of similar products.