On a basis of passively Q switched laser dynamical model, a rate equation of a passively Q switched intracavity optical parametric oscillator (OPO) was calculated and the dynamical process of internal OPO was discussed. The 1.57 μm parametric output with power of 15mJ is obtained and the one or several pulses were observed for different signal cavity reflectivity experimentally. The theoretical calculation was coincident with experiment results well.