Abstract: The cycloid ball planetary transmission system is a parametric vibration system with multi-freedom, and the dynamic characteristics are affected by the engagement rigidity excitation. Firstly, the translational-torsional coupling dynamic model of the system is established, which includes the factors such as the time-variant engagement rigidity and the bearing rigidity, and the dynamic equations of the system are derived. And then, the dynamic equations are converted to the canonical modal equations. The dynamic stability of the system is analyzed using the multi-scale method, and the combination resonance frequencies as well as the stability conditions are present. Finally, the steady-state response of the system is solved using the perturbation method. The results show that the parametric resonance is generated as the input rotational speed approaches the summation resonance frequencies; the system is stable as the input rotational speed approaches the difference resonance frequencies; the steady-state response of the system is the superposition of multi-response with different combination frequencies.