The laser beam automatic alignment system is applied to a high power laser facility for inertial confinement fusion. The far-field detection is a key technique of the automatic alignment system. Utilizing the image of spatial filter’s pinhole of the high power laser system and diffraction of sampling grating, a new method for far-field detection of laser beam is proposed and fully demonstrated on the experimental system. The experimental results indicate that, unlike the traditional method, this far-field detection system can fully use the space through off-axial grating sampling, and the average far-field alignment error is less than 0.9% of spatial filter pinhole diameter, which meets the alignment system requirements of less than 5% of spatial filter pinhole diameter.