Abstract: With the development of power system containing high percentage of both renewable energy and power electronic devices the trouble caused by sub-synchronous oscillation becomes increasingly prominent, so it has to be settled urgently to research the key influencing factors on the propagation of sub-synchronous oscillation in AC and DC transmission lines. For that reason, based on the system response measurement time series data, a quantitative analysis method for the key influencing factors of sub-synchronous oscillation propagation was proposed. Firstly, by use of the improved wavelet threshold noise reduction based on the adaptive noise complete ensemble empirical mode decomposition (abbr. CEEMDAN) the noise suppression of measured data was performed to reduce the affection of noise on the Prony analysis. Secondly, based on the correlation coefficient and mutual information of each influencing factor of subsynchronous oscillation propagation a combination model for correlation evaluation was established. Finally, by means of computing evaluation indices of different AC and DC parameters in comprehensive model, the key impacting factors of subsynchronous oscillation propagated in AC and DC transmission lines were obtained. Analysis results of a 2-region 4-machine system constructed in PSCAD show that the highly correlated parameter impacting the propagation of subsynchronous oscillation in AC transmission lines is the power flow in AC transmission lines and the highly correlated parameter impacting the propagation of subsynchronous oscillation in DC transmission lines is the impedance characteristics of AC transmission line under the subsynchronous oscillation frequency.