The ultra-low frequency oscillation is caused by the instability of the primary frequency regulation control process due to the negative damping of hydropower unit mechanical torque. In actual operation, the existing monitoring system cannot monitor the mechanical power of the unit, which causes the operator to be unable to identify the dominant unit and quickly calm down the oscillation. This paper analyzes the relationship between the damping characteristics of mechanical torque and electromagnetic torque, and proposes an online evaluation method of mechanical torque damping, which realizes the online identification of the ultra-low frequency oscillation dominant unit, and exits its primary frequency regulation to suppress frequency oscillation. The simulation results and the actual PMU recording curve verify the effectiveness of the proposed method which provides effective technical means for online control of ultra-low frequency oscillations.