The auto alignment system, which is used for the beam adjustment in high power laser system, is not only the key subsystem ensuring the routine operation effectually and safely, but also the key element involved in improving the far-field and near-field beam quality. Based on a skillful design of the grating and illumination-image system, a far field alignment system is realized. This system, which is based on a diffraction function of the grating, is characterized by the high performance and the simultaneous off-axis sample of the focus and the reference. The results, which are obtained when the system is used in pre-amplifier system of the SG-Ⅱ-Up, show that the reposition accuracy of the grating reference is better than 8μm, the jitter amplitude is less than 0.59 μm. After the alignment, the deviation of the beam focus from the grating reference is no more than 10 μm.