Passive mode locking of a Nd:YAG laer have been realized with a Cr 4+ :YAG saturable absorber and 190 ps pulses are obtained.According to this result,eht dvelopment and critical factors of passive mode locking are analyzed by fluectuations theory.Meanwhile,the action of kerr lens effects of Nd:YAG is discussed.And the results basically agree with our experiment.