An image blemish detecting system includes an image capturing module, a brightness adjusting module, and a blemish detecting module. The image capturing module captures an image. The brightness adjusting module adjusts the brightness of the image to obtain a second image having substantially uniform brightness. The blemish detecting module calculates a brightness ratio of each pixel in the second image, marks the pixels of which the brightness ratios are not smaller than a predetermined reference value as “1”, and marks the other pixels as “0”. The blemish detecting module calculates the quantity of the pixels in a continuous area in which all pixels are marked as “1”, and determines that the continuous area is a blemish if the quantity of the pixels in the continuous area is greater than or equal to the predetermined pixel quantity.