This paper summarized the mechanism of ultra-low-frequency oscillation (ULFO), studied that the negative damping torque of the hydraulic turbine governor is the major cause of ULFO. A Hamilton energy based method to evaluate contribution degree of hydraulic turbine to ULFO is proposed. Then, this paper presented a governor additional damping control strategy for suppressing ULFO. TLS-ESPRIT (total least squares-estimation of signal parameters via rotational invariance techniques) and improved chaos particle swarm optimization algorithm were used to optimize the controller parameters. Finally, the effectiveness of the contribution degree evaluating method and governor additional damping control strategy are demonstrated for an improved two-area power system.