Frequency oscillation belongs to the small disturbance stabilization problem of the active power-frequency (P-F) control process of the power system. The load with voltage regulation effect makes the reactive power-voltage (Q-V) control and P-F control coupled, so the dynamic Q-V control devices can also affect the frequency oscillation. The mechanism and process of dynamic Q-V control devices producing positive damping torque for frequency oscillation through supplementary control are analyzed. Strategy of supplementary damping control is proposed. For fast Q-V control devices with power electronic interface, a proportional link that takes the system frequency deviation as an input is added to the original Q-V control link to provide positive damping for frequency oscillation. A method for evaluating the effect of Q-V control devices at different positions on frequency oscillation is proposed. Selecting Q-V control devices with large effect for supplementary damping control can more effectively increase the frequency oscillation damping. The proposed method is verified with simulations in the New England 10-machine 39-bus system and an actual power system.