The Q switching characteristics of Cr 4+ :YAG crystal are studied theoretically and experimentally. The pulse energies and pulse widths of a Cr 4+ :YAG Q switched Nd 3+ :YAG laser in different conditions are measured. And the experimental results show good agreement with the theoretical predictions of energy and rough agreement with the theoretical predictions of pulse width. The other phenomena of the Cr 4+ :YAG Q switched Nd 3+ :YAG laser are also qualitatively analyzed.