A partial oscillation phenomenon in a digital-controlled three-phase inverter system, which is controlled by proportional controller in synchronous rotating reference frame, is analyzed in this paper. First, the discrete-time model of the system is derived. Then, a dynamics analysis scheme for this type of system is proposed. In addition, the underlying mechanism of the partial oscillation phenomenon occurring in this system is analyzed. The factors, which will cause the partial oscillation, are exactly predicted by analytical expressions. Finally, the theoretical results are verified by both simulations and experiments. These conclusions can provide an insight into the dynamics of the system.