Abstract: In view of the potential problem of harmonic current amplification of overhead transmission lines in three-terminal DC transmission system, a study is conducted on the harmonic current of bipolar overhead lines under different operating modes. Based on the theory of wave impedance, the symmetrical DC lines are simplified into the symmetrical component loops, and the unit length parameters are obtained. The calculation method for harmonic current of uniform line is analyzed. Based on the theory of wave refraction and reflection, a method for calculating harmonic current of hybrid transmission lines is proposed. By building the corresponding DC line model on the PSCAD/EMTDC platform for simulation, and comparing with the theoretical calculation results, it is shown that the proposed calculation method has a high accuracy in obtaining the line harmonic current under different operation modes of the three-terminal DC transmission system.