Abstract: Using FELOS code developed by us，the effects of short pulse and three-dimension are investigated.The spiking phenomenon was reproduced.Though there is so called “lethargy” effect during the early stage of FEL oscillation,the speed of spikes is always larger than speed of light in vacuum，after the length of synchrotron oscillation period equals to the one of wiggler.Comparing with one dimensional calculation，we point out that,the 3D effects destroy the bunching of electron beam，it results in washing out the periodic structure of optical spikes appearing in l-D simulation and makes the optical beam being nearly single spike of sub-picosecond length.It is stressed that sub-picosecond-scaledpulse will open new possibility of application of FEL.