Alignment accuracy of the cavity mirror in multipass amplifier is very high. Aiming at the requirements on alignment precision of cavity mirror in high power lasers, using image relaying of pinholes and the principle of beam automatic alignment, a scheme to align the cavity mirror is designed. It has been demonstrated on the simulation experiment system of the SG-Ⅲ prototype facility. The main amplifier's adjustment can be finished in 15min. The centering precision of the system is less than 0.5% of the beam diameter. The pointing precision of the system are less than 0.3".