With the widespread adoption of new energy sources like wind and solar energy, the new power system exhibits typical "double-high" characteristics, and as a result, sub-synchronous oscillations are becoming an increasingly significant issue. It is vital to research the influencing factors and propagation mechanisms of sub-synchronous oscillations in complex AC-DC power grids since they have a wide area of propagation characteristics. This paper proposed an evaluation index system of sub-synchronous oscillation propagation influence factors based on the maximum information coefficient (MIC), and the propagation mechanism of sub-synchronous power propagation based on diversion coefficient is elaborated according to the derived strong correlation influence factors. The results indicate that the AC line power flow and the AC line impedance characteristics at the sub-synchronous oscillation frequency, which correspond to the AC component and the DC component of the sub-synchronous power. are the strongly correlated factors affecting the propagation of sub-synchronous oscillation. In the complex AC-DC grid, the sub-synchronous power propagates determined by the proportion of the diversion coefficient. The efficiency of the approach and mechanism suggested in this article is confirmed by the simulation results.