This paper analyzes the DC resonance issue of a hybrid double-terminal HVDC system by simplifying the DC side of the MMC into a series-connected LC load and establishing mathematical model of the system. In order to suppress the DC resonance, a new control method is proposed to inject damping into the resonance system. As a result, the DC resonance is eliminated by dynamically adjusting the total number of inserted sub-modules of the MMC, without changing the current and voltage on the AC side. The feasibility and performance of the proposed method are validated by simulations in PSCAD. The control method proposed in this paper can eliminate DC resonance issue of a hybrid HVDC system.