LCC and VSC are essentially different in the steady-state model and working principle. When the two converters are mixed and applied to the same multi-terminal HVDC transmission system, the steady-state mathematical model and the power flow calculation method have also changed accordingly. Firstly, the steady-state model of the hybrid system is described. Then, based on the comprehensive consideration of the working principle and control mode of the two converters, the mathematical model of the power flow calculation of the hybrid system is derived. Furthermore, an alternating solution algorithm suitable for the hybrid system is proposed. Finally, the effectiveness and accuracy of the proposed algorithm are verified by a hybrid DC transmission system example and a hybrid three-terminal system.