The laser beams automatic alignment system is acyusting to adjust beam lines accurately in the high power laser facility for inertiai confinement fusion. Considering the requirements on alignment time and alignment precision of the SG-III prototype facility, using techniques on image fiber and transmission fiber, combining the image relaying principle of near field and far field, innovating in cavity mirror adljustment by using inserted fiber light source, utilizing the characteristic of image relaying in four-pass amplifiers, a reasonable and optimized scheme for automatic aligning four-pass beam paths is presented It has been demonstrated on the simulation experiment system of the SG-III prototype facility. Results indicate that beam paths acyustment can be finished in 15 minutes by the automatic alignment system. The precision of the near field acUustment was less than 0.5% of the spot The accuracy of the far field adljustment was less than 0.3". Requirements of the SG-III prototype facility are met.