A blemish detecting system includes an image capturing module, a brightness calculating module, a blemish detection module, a storage module, and a processor. The image capturing module captures an image. The image has a number of pixels. The brightness calculating module calculates the brightness values of the pixels. The blemish detection module gives serial numbers to the pixels according to the locations of the pixels, scans the pixels according to a first sequence, and marks the pixel of which the brightness value is less than a predetermined brightness value as a blemish. The storage module stores the serial numbers of the scanned pixels, and thus the blemish detection module does not need to scan the pixels for a second time. The brightness calculating module and the blemish detection module are computerized instructions and are executed by the processor.