The closed-loop control of system position and speed is realized by co-operating of photoelectric shaft encoder, CPU board, I/O interface board and serial port board in a servo system. When there is a failure in any parts of photoelectric shaft encoder channel, the entire system will be paralyzed. A test equipment is designed to locate the failure point quickly. It is important to debugging operation, especially when there is a failure in the servo system operated in field environment, the failure can be located and solved quickly, which has stronger practicability.