A method for monitoring large deformation of a surrounding rock of a tunnel based on an automatic targeting and ranging system includes: acquiring, by a camera module, a target monitoring image; identifying a center image position in the target monitoring image; performing, by a laser ranging module, an automatic targeting and ranging task according to the center image position; and calculating a deformation offset of each target of left wall targets, right wall targets and a vault target based on an automatic targeting and ranging result corresponding to the target. The method solves the problem that a laser ranging device fails to measure a target at a fixed angle and obtains a real and accurate deformation offset thereby greatly improving the reliability of the large deformation monitoring result of the surrounding rock of the tunnel.