The stage of precision machining center will deform under the weight of large workpiece which is put on it. The deformation will cause the workpiece to incline, and moreover, affect the machining accuracy, which cannot be neglected especially in large-scale combined precision machining centers. To resolve this problem, after simulating the deformation of the stage, this paper introduced an automatic leveling method for the precision stage and gave a method to calculate the regulating variables of each supporting point of the stage. In order to verify these methods, an experiment was done on an experimental stage which was designed and built on the basis of the stage of precision machining center. Experimental results showed that the tilt error of the stage can be reduced with these methods and the machining error of the workpiece declined by 90% after leveling. The experiments also verified the accuracy and feasibility of the automatic leveling method.