An investigation was made of the formation of ultrashort pulse trains in an YAG laser,and the effects on the passive mode locking process of the initial dye absorption coefficient x_o,the saturable power of the dye solution I_s,and the ratio of the beam cross section in the laser medium to the beam cross section in the absorber.The laser under investigation produces bandwidth-limited output pulses at 1.06μm,with spectral profile and temporal profile in Gaussian function.Single pulses can be adjusted from 20ps to 200ps.