For the analysis of the coupling characteristics of the power system frequency dynamic and power angle oscillation,the mechanism of the two-machine system under disturbances is derived in this paper, and the mechanism of the multi-machine system is obtained based on the extended equal area criterion (EEAC). A quantitative assessment index is proposed based on the Pearson coefficient,which quantified the coupling according to the characteristics of frequency dynamics and power angle oscillation. The influence of power angle oscillation on the frequency dynamic indices is analyzed,and the coupling degrees of between the power angle oscillation and different frequency dynamic indices are assessed quantitatively. The results of the mechanism analysis and case study verify the proposed quantitative indices and show the coupling characteristics of the power system frequency dynamic and power angle oscillation,which provided guidance for optimal frequency and power angle control.