A system and method for automatically focusing an optical lens controls the light generated by a light-emitting device of an image measuring machine to penetrate a glass sheet, so as to project a picture of the glass sheet onto an object. The system and method further moves an optical lens along a Z-axis of the image measuring machine to capture one or more digital images of the object, and computes a definition value of each captured digital image. Furthermore, the system and method obtains a focus position corresponding to the highest definition value of the captured digital image.