Abstract: For the imbalance of power allocation when the loads changes in the Multi-terminal HVDC transmission system, a DC voltage slope control strategy was proposed. The control strategy uses hierarchical control, including processing layer and implementation layer. In the processing layer, a mathematical model of unbalanced power allocation was established, namely the OUPA mathematical model. The minimum of the extra generation cost was set as the objective function, and the constraints include the line capacity, generator capacity, converter station capacity and power balance. The optimal solution of the model was gained by applying Lagrange multiplier method. And then the unbalanced power was allocated optimally by implementation layer. A five-terminal VSC-MTDC simulation model in the Matlab/Simulink platform was built in the article, three situations have been simulated and discussed. The simulation results show that the proposed control strategy can reduce the extra generation cost greatly and when faults happen, the DC voltage can be stabilized and maintain the power balance. Besides, the control strategy has the advantages of simple structure, strong robustness and easy to implement.