Based on the rate equations for passively Q-switched four-level lasers, and by introducing the concept of optical parametric oscillator (OPO) effective conversion cross-section, a model for the passively Q-switched single resonant intracavity optical parametric oscillator (IOPO) is built. The parametric gain expression which taking account of the absorption loss of the material, walk- off effect, mode matching, etc., and also the OPO effective conversion cross- section expression are deduced. By using the Runge- Kutta method, the dynamic characteristics of the IOPO are simulated. Moreover, the buildup process of the signal pulse is discussed, and the impacts of the signal reflectivity on the energy conversion process and on the buildup time and pulse duration of the signal pulses are analyzed. The simulation results reveal that there is an optimal value for the signal reflectivity which is 0.85. Besides, the simulation results and experimental results are compared, and they match very well. The findings provide a theoretical basis for understanding the performance mechanism and optimization of the passively Q-switched single resonant IOPO.