The invention discloses a method for manufacturing a large-size ultra-thin laser crystal, and belongs to the technical field of crystal manufacturing. The method comprises the following steps: uniformly coating a little water on a non-viscous side of a vacuum adsorption pad; putting a crystal to be manufactured on the non-viscous side; extruding the vacuum adsorption pad to completely discharge water to form a vacuum belt and ensure that the crystal and the vacuum adsorption pad are adsorbed firmly; adhering the viscous side of the vacuum adsorption pad on an aluminum tray; mounting the aluminum tray on a biaxial machine; manufacturing the surface of the crystal to be manufactured according to the requirement; detecting the surface shape of the crystal when the manufacturing is completed and the crystal is cooled to room temperature; and taking the crystal off the tray. The ultra-thin laser crystal manufactured by adopting the method ensures that the surface shape after taking off thetray cannot deform; and the surface shape of the ultra-thin laser crystal manufactured by the method can achieve lambda/10 to lambda/4, and the smooth finish can reach 20/10.