We report on a passively Q-switched quasi-cw diode-pumped Nd: YAG including an intracavity optical parametric oscillator. The dynamics of this system is described by solving the coupled equations. The effect of the initial transmission of Cr4+ :YAG saturable absorber on the signal wave operation is studied. Under optimum conditions,we achieve 2.3 mJ energy at 1.57μm wavelength for 40 Hz repetition rate. The peak power of the pulses amounts to 0.88 MW with the pulse width of 2.6 ns. When the Fresnel reflection losses of the filters are taken into account,the pulse energy would be higher than 2.3 mJ. To the best of our knowledge, this is the highest pulse energy and peak power for such a type of single resonant quasi-cw diode pumped Nd:YAG/Cr4+ :YAG IOPO laser.