A method for detecting defects in camera modules is provided. The method includes the following: an image is acquired from the camera module; a comparison formula and a standard value of defect luminance are set; the image is divided into many test regions; the corresponding reference regions are then plotted out; a test region is selected, and a reference region is confirmed correspondingly; averages of gray scale values of the selected test region and the confirmed reference region are calculated; a defect luminance of the selected test region is calculated; the calculated defect luminance is compared with the standard value for confirming whether the camera module is of satisfactory quality. A related system is also disclosed.