Single pulses at different time have been selected from a mode-locked pulse train produced in a passive mode-locked Nd:glass laser,and the temporal and spatial characteristics of the pulses have been studied.Experiments show that bandwidth-limited pulses with Gaussian temporal and spatial distribution appear in the front part of the pulse train,the spectral width is broadened significantly and obvious modulation appears with the evolution of the train.The broadening is in good agreement with theoretical calculation results.It proves that the broadening is caused by self phase modulation under the action of intense light.Some experimental phenomena are discussed.