For a high-voltage DC (HVDC) transmission system, this paper describes an improved algorithm for the coupled multi-phase transmission line model and complete procedures for the calculation of the DC-side harmonic currents. First, based on the phasemode transformation, an improved coupled multi-phase transmission line model is proposed. In other words, the transmission line is represented as a nodal admittance matrix form, considering computational accuracy and efficiency. In addition, when calculating the DC-side harmonic currents, the line-commutated converters at the rectifier side and the inverter side are equivalent to the three-pulse harmonic voltage source models. Second, based on the DC-side equivalent models mentioned previously and the nodal voltage analysis method, an analytical method and complete procedures for the calculation of the DC-side harmonic currents in the HVDC systems, namely, the equivalent disturbing current, are described. Finally, a case study is carried out on a 4000 MW/±800 kV HVDC system. In the same condition, the analytical results are basically consistent with the simulation results of PSCAD/EMTDC, thus proving the accuracy of the proposed model.