Based on the engineering practice of Yunnan Power Grid, the 4-machine 2-area system model including DC transmission is established. The key issues of the modal analysis method applied in the frequency mode are summarized. The influences of governor model, DC model and operation mode on frequency mode are analyzed. The frequency domain and time domain characteristics of frequency mode and power angle mode are compared. Furtherly frequency mode is dominated by the governor and prime mover and appears as a coherent change in frequency. The angle mode is dominated by the PSS and the excitation system and is represented by the relative sway of the angle. If the system has both negatively damped frequency mode and angle mode, it will appear as the beat frequency oscillation phenomenon of frequency and angle. Finally, the modal analysis method is used to identify the different oscillation modes in the large power grid, to guide the adjustment of the units’ parameters. The analysis results provide technical support for revealing the oscillation mechanism of large power grid and formulating suppression measures.