For the effective suppression of ultra-low frequency oscillation in Northwest power grid with a large proportion of hydro power in China, it is crucial to simultaneously optimize multi-stage frequency control parameters of speed governing system and local control unit. To address this problem, a parameter optimization method for the multi-stage frequency control of hydropower is proposed to suppress the ultra-low frequency oscillation in this study. Based on the multi-stage frequency control model, the security region of joint control parameters for suppressing ultra-low frequency oscillation is theoretically analyzed. Moreover, with the goal of minimizing the absolute value of comprehensive deviation multiplied by the time integral in the security region, the multi-stage control parameters are optimized through particle swarm optimization using the Matlab/Simulink platform. Finally, the time-domain simulation based on the PSCAD/EMTDC platform verifies the effectiveness of theoretical analysis and optimization method of multi-stage frequency control parameters for suppressing the ultra-low frequency oscillation of hydropower.