Beam automatic alignment system is an important sub-system used in high power laser inertial confinement facility for beam adjustment. Far-field alignment is a key technique of the automatic alignment system. Based on the character of diffraction grating, the new far-field alignment system can sample far-field flexibly. The main factors that affect the precision of the far-field detection system are analyzed. The result shows that the systemic error which includes projection error and aslant imaging error is 7.49% of the spatial filter pinhole, and stochastic error which includes object-image error and diffraction light suited error is less than 1% of the pinhole.