Trichel pulse is an interesting self-pulsing phenomenon in DC negative corona discharge.It was considered to occur only in electronegative gas and the behavior of negative ions is responsible for its formation[1,2].In this work,we investigate the developing process of Trichel pulse in air by recording the time-resolved ICCD images.The needle-to-plate configuration is employed to produce the negative corona in ambient air.The voltage on the needle and the discharge current are monitored by an oscilloscope.The time-resolved ICCD images and the current waveform are shown in Fig.1 at average current of I =120 μA.