Abstract: For testing large aperture telescope with a segmented primary mirror, a low-coherence-spectrum interferometric detection system based on the Michelson interferometer principle was put forward to detect the phase error between the “sub-mirrors”, and subsequently correcting the misaligned segments to achieve the mirror coplanar. The phase error between the segments was extracted from the interferograms mismatching. The physical structure of the interferometric detection system with the low-coherence-spectrum was presented, and the detection principle of the interferometric detection system was explained. The interferometric fringe contrast and SNR were analyzed for the interferometric detection system. The feasibility of detecting the phase error with the low-coherence-spectrum interferometric detection system was demonstrated .