Multi-terminal HVDC system has attracted much attention due to its flexible structure and high economic efficiency. Because the operation mode of multi-terminal HVDC system is flexible，the selection of key equipment parameters and the calculation of the main circuit state need to consider more factors. Firstly，the operating conditions of the multi-terminal DC system，the operation mode and control mode of the multi-terminal DC system are analyzed. Then，the limiting factors of the key equipment parameters are analyzed，and the computation method for the key parameters of the multi-terminal HVDC are proposed. The power flow calculation equation of the DC network is constructed，and the adjustment method of constraint conditions is proposed. Finally，the validity of the proposed method is verified by design examples. The method proposed in this paper provides technological means and support for the design of multi-terminal HVDC systems.