A measuring system comprises a pulse generator, an under test device, a variable resistor and a detecting control system. The pulse generator provides pulse signals with different voltage peaks to the under test device and the variable resistor. The variable resistor adjusts its resistance value according to a control signal. The detecting control system detects the voltage ringing ranges of the first terminal of the under test device at different resistance values. The detecting control system generates the control signal to adjust the resistance value of the variable resistor according to the voltage ringing ranges.