During the process of nano-lithography on solid material surfaces by a Scanning Tunneling Microscope (STM) system working at constant current mode, while a high-voltage electric pulse acts on the gap between the tip and the sample surface, the feedback circuit causes the tip to withdraw swiftly, resulting in a reduction of effective pulse-width. In the experiment presented in this paper, a specially designed and computer controlled temporary "holding" function can maintain the tip position and the tip-sample distance during the whole period when the highvoltage pulse acts, so that the effective pulse-width can be accurately measured. An experiment of lithography on a graphite surface in air found, for the first time, the threshold of effective pulsewidth to be (0.04±0.01) µS, with a voltage amplitude of 4 V.