A measuring method of the longitudinal mode of rep-rate pulsed laser based on confocal spherical sweeping interferometer is demonstrated.This method can not only examine whether the laser is single longitudinal mode or multiple longitudinal mode,and calculate the distance of longitudinal mode and the width of spectrum,but also tell the energy distribution of each mode.It is highly precise,intuitionistic and convenience.The measuring of longitudinal mode of pulsed laser with over 1kHz rep-rate is achieved.The picture on the oscilloscope is clear and steady.There is no similar paper seen in domestic or overseas literature.