A computer-based method for adjusting luminance of a light-emitting device on image measuring machine is provided. The method includes reading a model definition curve and model coordinates of an object and a charge couple device (CCD). The method further includes locating the object and the CCD to positions on the image measuring machine, and capturing a digital image of the object. Furthermore, the method includes adjusting a resistance of the light-emitting device to ensure an ordinate deviation corresponding to each abscissa value between a new definition curve and the model definition curve falls in an allowable deviation range. A related system is also provided.