As to the identification and calibration of index-instrument, a novel method for reading recognition based on digital image processing is proposed. Firstly, preprocessing operation on images is done, including image segmentation, image filtering, image dilation and thinning etc, and a method of improved adaptive median filter is adopted to improve the image. Then, characteristic information of image is extracted with the improved dual thresholds Hough transformation, and the pointer reading is got using the linear relationship between scale and angle of instrument dial revolutionary. And as to the situation that pointer coincides with the whole tick, late correction has been done. The experiment results show that the algorithm is very valid for the pointer angle recognition, and has higher precision and real time. It has strong practical application value.