In view of the issue that in the process of locomotive motor commutator maintenance and processing, laser modulation and positioning type mica slot engraving system are widely used in real applications. However, it has low precision and requires a lot of human intervention. To overcome this problem, this paper proposed a precise positioning method of mica slot based on machine vision, which can accurately extract the edge of the motor commutator. The system first independently designs the precision compensation algorithm and constructs the positioning error correction model. Based on this, the embedded system platform is built to realize the automatic, rapid and accurate positioning the center line of the mica slot. Then the system achieves accurate calculation the deviation between the knife and the center line and control the bit tool to move to the correct engraved position. Experimental results indicate that the instrument can accurately calculate the position of the mica slot center line. Through the servo motor control burin to adjustment and aim the midline, the knife positioning error has been controlled between the positive and negative 0.02 mm, achieving the entire operation process automation and precision.