This paper builds equivalent circuit of multi-terminal DC network composed of different types of converters and multi-terminal DC lines, and proposes the control methods of different types of converters and the coordination mechanism of control modes among converter stations in multi-terminal HVDC transmission systems. A general electromechanical transient model of multi-terminal DC transmission system is established, and the model is realized based on the AC/DC power system calculation and analysis software (DSP). The results of the model is compared with PSCAD model through an example system. The comparison results show that the response characteristics of the established multi-terminal DC system model are in good agreement with that of the PSCAD model, and the accuracy of the model is verified.