In order to address the issues that the performance of instrument identification system in intelligent patrol robot is affected by the varying illumination, and has the low identification accuracy, instrument identification system with high accuracy is built according to the characteristics of pointer instrument image from high voltage electronic field. In view of the outdoor work environment of patrol robot, iterative Otsu algorithm is proposed in the paper, which can achieve the extraction of pointer area in instrument image under all kinds of illuminations. After the analysis of pointer rotation mechanism and image characteristic, pointer angle recognition algorithm based on Hough transform is proposed in the paper, the function between pointer angle and reading is deduced. Compared with the traditional pointer angle extraction algorithm based on Hough transform, the constraint condition of pointer center line through the center is added in the algorithm, which can increase the accuracy of angle extraction, and reduce the amount of search data and time. The proposed instrument identification system in the paper can achieve more than 95% of the correct recognition rate under all kinds of outdoor illumination. Experiments show that the system proposed in the paper is robust to illumination condition, pointer width, dial interference, imaging angle without the shadow of pointer. However, when the shadow of pointer is generated due to imaging angle, the extraction of central line and the calculation of angle are biased, then the accuracy of reading recognition is reduced.