Pulse shortening is an universal phenomenon in highpower microwave tubes, which hinders the improvement of microwave output energy. So far, it is also an unres olved problem in the field of high-power microwave devices. In this paper, the relativistic backward wave tube (RBWO) is treated as an example to study the pul se shortening phenomena. The influences of explosive emission in the inner surface of RBWO and fluctuation of the relativistic electron beam on the pulse width and output power are investigated by means of the particle in cell(PIC) method .Through the simulation results, some useful conclusions have been drawn. The explosive emission in the surface of the slow wave structure due to intense elec tric field is one of the most important factors causing pulse shortening in high-power microwave tube. The fluctuation of the electron beam can also lead to pulse shortening. Some methods to overcome pulse shortening are given in this paper.