The invention discloses a high precision leveling method and a high precision leveling device for a large optical element. The method includes: using a high-power microscope to perform continuous multiple dark field grey level image acquisition to three characteristic points at different positions on the surface of the optical element, calculating image grey level information entropy, fitting thegrey level information entropy with an axial moving distance curve of the microscope, searching for a minimal value in the curve to obtain a focusing position, and obtaining defocusing amount of eachcharacteristic point. The leveling device is in a two-plate and elastic pre-tightened design. Pitching and lateral swaying of the leveling device are adjusted through two points. By building a conversion model of the defocusing amount and adjusting amount to convert the defocusing amount of the three characteristic points into the adjusting amount of the two adjusting point, leveling operations can be performed. The problem of defocusing of the large optical element due to non-parallelism of the optical element and a focal plane of the microscope during a full-range scanning and detecting process using the microscope is solved.