A method for measuring the non-orthogonality angle of a square mirror and the correction value of the scaling factor, comprising, through a photolithography machine system, exposing the marked pattern A on the mask to a glue-coated blank silicon wafer according to the specified pattern distribution, and exposing After the completion, unload the film; rotate the silicon wafer at a certain angle and reload the film, and then expose the marking pattern B on the mask to the silicon wafer according to the same pattern distribution, and the position of the exposure mark is biased relative to the first layer. After the exposure is completed, develop; load the film again, but do not rotate, and after the off-axis alignment system of the lithography machine aligns the marks on the silicon wafer, read all the mark positions; calculate according to the measured mark position Obtain the non-orthogonal angle and scaling factor of the workpiece table coordinate system. This measurement method is an absolute measurement method, which is applicable to different lithography machine systems.