Using a xenon flash lamp as the pump source, Cr4+: YAG as a passive Q-switch, we have realized the green laser output at 0.53 μm from a self-frequency-doubling NYAB crystal. The green laser output energy, pulse width and repetition rate under conditions of different cavity lengths and small-signal transmission of Cr4+:YAG were measured. Meanwhile, Cr4+:YAG Q-switched coupling wave rate equations for NYAB crystal were given and numerical solutions of the equations were in agreement with the experimental results.