The gear transmission system is used to transmit the traction motor torque to the axle, and during the running process it may have a relaxation state. With the comparison of the traction motor speed and the wheelset speed during the traction, electric braking and air braking process, the relaxation state of the metro train gear transmission system could be diagnosed. The proposed method has the advantages of utilizing the existing vehicle system equipment without additional equipment, and has been applied on a domestic subway line.