The optical axis parallelism was an important parameter for the multi optical axis optoelectronic device. It was very necessary to detect the parallelism. With the gradual increase in the complexity of optoelectronic devices, the distance between optical axes was increasing. The traditional small aperture detection methods could no longer meet the requirements of existing photoelectric devices. In order to detect the parallelism between large space optical axis, a expanding component based on double pentaprism structure was designed to realize the translation of the beam emitted from the collimator, and the error of the expanding component was modified by double wedge structure. The effective aperture of the collimator was extended from 300 mm to 1 200 mm. Mathematical modeling analysis and actual adjustment of the expanding component were carried out and the precision experiment was done. The experimental results of the detecting system show that the beam emitted by the collimator could maintain good parallelism after expanding. The parallelism deviation is within 11″, which could meet the precision requirements of large space optical axis detection.