DC voltage stability is one of the most important issues for reliable operation of VSC based HVDC system. In this paper， principle of the direct current control and DC voltage error method used in VSC based HVDC system is analyzed. A multi-point DC voltage control strategy based on DC voltage error method is then proposed for VSC based multi-terminal HVDC system. This approach can realize automatic switching between active power and DC voltage control modes， so that control of DC voltage and active power balance can still be realized after lost of the converter which is in DC voltage control mode. Simulation is carried out on a VSC-MTDC system with five terminals and the results show that reliability of the multi-terminal HVDC system is improved with the proposed control strategy and satisfactory dynamic characteristics can be obtained.