An optical critical dimension measuring method, applicable in measuring a pattern, that includes a plurality of polysilicon layers, of a device, is provided. The method includes obtaining a real curve corresponding to the to-be-measured device. Then, determining whether an ion implantation process has been performed on the polysilicon layers, a different module is selected. A correlation process is performed according to the selected module to generate a theoretical curve that correlates with the real curve to obtain a plurality of parameters corresponding to the theoretical curve.