Abstract: Focusing on the ultra-low frequency power oscillation phenomenon caused by hydraulic transients of the tailrace tunnels in the operation of a large hydropower station, this paper investigated the hydraulic characteristic modeling of the free surface-pressurized tailrace system in the hydropower station and the ultra-low frequency oscillation suppression schemes of the unit. First, by introducing the equivalent circuit theory to hydraulic components modeling, the equivalent circuit model of the open channel was derived by analogizing to that of the pressurized pipe. Then, the equivalent circuit topology of the overall free surface-pressurized tailrace system with one-tunnel for two-machine layout was established. The hydraulic pressure's periodic fluctuation at the tailrace gate under relatively low downstream water level condition was simulated on this basis. Besides, the influences of the factors such as downstream water level and the open channel local hydraulic loss to the ultra-low frequency oscillation had been analyzed. Furthermore, the feasibility and relative merits of the two oscillation suppression approaches via respectively increasing the local hydraulic losses on the branch tailrace tunnel or the main tailrace tunnel were discussed. The field tests had indicated the evident oscillation damping performance of the proposed scheme.