An apparatus for ring-back constriction is provided. The apparatus includes a comparator, a termination controller, a termination variable resistor, a constriction controller, and a constriction variable resistor. The comparator is coupled to the transmission line, for comparing the line signal with a reference voltage, and accordingly outputting a comparison signal. The termination controller is coupled to the comparator, for outputting a termination control signal to adjust the resistance of the termination variable resistor. The constriction controller is coupled to the comparator for outputting a constriction signal to adjust the resistance of the constriction variable resistor. The ring-back effect is constricted due to impedance matchtermination voltage when an undershot occurs.