Dependences of ground state absorption and excited state absorption of Cr 4+ :YAG upon fluence and intensity of input laser pulses are calculated based on the rate equation. Measuring transmission of this crystal, we discovered that its excited state absorption tended to be saturated. Key parameters are obtained by matching the theory calculation with experimental data. The Nd:YAG laser with 5 ns pulsewidth was operated and found which is directly relevant to excited state absorption saturation of Cr 4+ :YAG .