Automatic alignment of laser beam is an important part of the large laser device. Taking classical imagerelaying unit in an excimer (XeCl) laser master oscillator power-amplifier (MOPA) system as an example, the establishment of alignment benchmark, the automatic alignment principle of 3-ray and charge-coupled device (CCD) image acquisition and processing method are introduced. It has been proved by the experiment results that this alignment unit can appease the request of laser automatic alignment of MOPA system.