For the case of a Q switched Nd∶YAG laser with a KTP parametric oscillator inside the laser cavity, the temporal evolutions of the population inversion and of the pump and signal intracavity power are discussed by using the simple model of coupled equations. The results show that the optical parametric oscillator can produce single or multi pulses, depending on the signal cavity finesse and the pumping level. The single and multi pulses were obtained by the different signal cavity reflectivity experimentally. The theoretical calculation was coincident with experiment results well. The characteristics of intracavity optical parametric oscillator are analyzed.