In this paper single ultra-fast voltage pulses are introduced to the Pt/Ir tip of a scanning tunnelling microscope (STM),and the non-destructive threshold of the graphite surface is studied systematically in a wide range of pulse durations(from 104 to 8 ns).Considering the waveform distortion of the pulses at the tunnelling region,this paper gives the corrected threshold curve of pulse amplitude depending on pulse duration.A new explanation of threshold power has been suggested and fits the experimental results well.