The operation stability of renewable power system presents new forms and characteristics, and subsynchronous oscillation is one of the representative problems. The influence factors of subsynchronous oscillation propagation path are complex. This paper studies the subsynchronous oscillation power and its propagation characteristics, and proposes an algorithm for the dominant component of subsynchronous oscillation power. The algorithm starts from the complete definition of subsynchronous oscillation power, characterizing the propagation of subsynchronous oscillation by the amplitude of AC component and analyzing the propagation characteristics afterwards. The proposed algorithm decouples the power frequency component and subsynchronous oscillation component in the expression through mathematical derivation. Finally, the effectiveness of the algorithm is verified by a two-machine five-node case.