With the continuous advancement of flexible direct current (DC) asynchronous interconnection projects, the ultra-low-frequency oscillation problem caused by the high proportion of hydropower poses a threat to the frequency stability of the power system. To suppress the occurrence of these oscillations, an additional frequency control strategy based on a flexible DC transmission is proposed, which is also referred to as a voltage source converter-based high-voltage DC transmission (VSC-HVDC) system. Firstly, a frequency response model that includes both hydro and thermal power units is constructed. The formation mechanism of ultra-low-frequency oscillation is identified through damping torque analysis, and the influence of various governor parameters and different hydropower ratios on the occurrence of ultra-low-frequency oscillations in the system is explored. Secondly, based on the coupling characteristics of DC capacitor voltage and alternating current system frequency, the mechanism of the converter station participating in suppressing ultra-low-frequency oscillation is studied. A control method for virtual inertia and VDC-f droop of the converter station is proposed, and the virtual inertia and droop parameters are designed under constraint conditions. Finally, based on the MATLAB/SIMULINK platform, a load frequency control model that includes both hydro and thermal power units is constructed, and the effectiveness of the proposed method is verified by comparing the optimization methods of water turbine parameters.