During the transients caused by the AC faults or load rejections， the DC link of voltage source converter based high voltage DC（VSC-HVDC） system may experience under-voltage or over-voltage because of the active power unbalance， which endangers the safety operation of the valves， DC capacitors and cables. To keep the DC voltage in the safety limits under the steady and transient conditions， a novel control strategy is proposed. It introduces a DC voltage dependent current order limiter（VDCCOL） into the outer active power controller. VDCCOL changes the inner active current orders according to the DC voltage fluctuations. When the DC voltage is abnormal， VDCCOL rapidly modifies the outer control loop current setting points according to pre-set characteristics， to mitigate the active power unbalance， therefore the DC under-voltage or over-voltage is suppressed and the risk of tripping the whole system is reduced. Simulation results show that the proposed control strategy can effectively suppress the variations of the DC link voltage during the transients and enhance the system's operational characteristics.