With the large scale VSC-HVDC projects put into operation,the black start scheme for the power grid to meet the needs of dispatching operation department is urgently needed for the practical project application of the black start based on VSC-HVDC. In this paper, the mechanism of the black start control strategy and the feasibility of the black start with the target network charging are studied. In view of the current control strategy, the voltage and current surge occur in the charging process, which makes the converter trip and black start fail in severe cases. An improved black start control strategy is proposed to optimize the time sequence of the switch closing, and the VSC-HVDC system black start with target network stepping up from zero voltage is achieved. Meanwhile, a black-start simulation model using Yunnan back to back MMC-HVDC system as the external power supply of Yunnan Power Grid/Guangxi Power Grid based on RTDS is built. By using the I/O device connecting the VSC-HVDC control and protection system and the RTDS model, a closing loop simulation platform of power grid black-start system is built. Simulation results verify the effectiveness of the proposed improved strategy.