The invention discloses a closed loop muscle relaxation iatrical effect monitoring and injecting method and a closed loop muscle relaxation injecting device. The method comprises the specific steps of: acquiring muscle myoelectricity feedback actions of a patient through an independently developed muscle relaxation monitoring piezoelectric inductor and operating to obtain a muscle relaxation monitoring data TOF (turnover of frequency) value; and controlling a stepper motor to operate at a specified speed so as to push an injector to administrate medicine at a set speed by using the TOF value as a feedback data base and by a microprocessor by combining a built-in muscle relaxation administration method. The scheme can monitor the muscle relaxation state of a patient in real time, objectively reflect the true iatrical effect condition of the muscle relaxant in the human body and display the TOF value of a muscle relaxation feedback value, and meanwhile, an injection system can automatically administrate the muscle relaxant in the whole course according to the TOF value. Moreover, artificial interventions are not needed, and the automatic administrated dosage and the method are obtained through a large quantity of population parameters. The invention ends the times of fuzzy muscle relaxant administration and can scientifically improve the controllability of the muscle relaxant, avoid medicine deficiency or excess and improve the operation quality.