The invention belongs to the instrument graduation line reading technology field and more particularly, discloses a graduation line reading automatic detection method based on a projection method and comprises the following steps: 1) matching a graduation bar template image with a current image and positioning the graduation bars; 2) performing characteristic training to the graduation bar colors and based on the color training result, carrying out the color confidence calculation to the current image; 3) based on the horizontal projection, calculating the inclined angle and correcting the image; 4) based on the vertical projection, positioning the position of the graduation line; and 5) identifying and determining the graduation bars which are of dark graduations, bright graduations or no graduation and making statistics about the total number of the graduation bars. According to the invention, the instrument graduation disc image can be processed on the basis of the projection method, which realizes the accurate and automatic reading of the instrument graduation disc's graduation line.