This letter studies large-disturbance stability of the power system with a synchronous generator (SG) and a converter-interfaced generation (CIG) connected to infinite bus. The power system is multi-timescale and first simplified. It is shown that the boundary of region of attraction (ROA) of the simplified model is composed of stable manifolds of unstable equilibrium point (UEP) or semi-singular point (SSP), named anchor points, and singular surface pieces. The type of anchor point determines the dominant instability pattern of the power system. When the anchor point is UEP or SSP, the dominant instability pattern is the instability of rotor angle of SG or the instability of phase-locked loop and outer control loop (OCL) of CIG, respectively. Transition of dominant instability pattern can be analyzed with the relative position relationship between UEP and SSP. The effect of OCL is discussed. When the OCL is activated, the ROA becomes smaller and the system is more prone to instability of CIG. It is necessary to consider the OCL when studying the large-disturbance stability of the power system.