The possibility of using Cr4+: YAG as saturable absorber to achieve passively mode-locked operation in a Nd: YAG Laser is proposed theoretically. From the rate equations, we derived the resumption time and the saturable light intensity of the excited-state of Cr~4+: YAG acted by strong laser pulses. Passively mode-locked operation is experimentally realized by using Cr~4+: YAG as saturable absorber in a pulsed Nd: YAG laser with a nearly critical stable resonator and an antiresonant ring. The pulse train with energy of 13.5 mJ and pulse duration of 180 ps are obtained.