In this paper, the rate equations describing the operation of intracavity-pumped Q-switched terahertz parametric oscillators based on stimulated polariton scattering are given for the first time. The rate equations are obtained under the plane-wave approximation, the oscillating fundamental and Stokes waves are supposed to be round uniform beam spots. Considering the fact that the terahertz wave nearly traverses the pump and Stokes beams and using the coupled wave equations, the terahertz wave intensity is expressed as the function of the fundamental and Stokes intensities. Thus, the rate equations describing the evolution processes of the fundamental and Stokes waves are obtained in the first step. The THz wave properties are then obtained. Several curves based on the rate equations are generated to illustrate the effects of the nonlinear coefficient, the THz wave absorption coefficient, and pulse repetition rate on the THz laser characteristics. Taking the intracavity-pumped Mg:LiNbO3 TPO as an example, the THz frequency tuning characteristic and the dependences of the fundamental, Stokes, and THz wave powers on the incident diode pump power are calculated. The theoretical results are in agreement with the experimental results on the whole.