Abstract: In this paper,the difficulty of calculating the DC side harmonic currents in a double circuit on the same tower HVDC system was analyzed.Since the symmetrical component method was very difficult to apply in the double circuit on the same tower HVDC transmission lines,the universally applicative multi-phase coupled transmission line model and its calculating method was adopted,and this method was improved by changing the series expansion form of the transmission line admittance matrix,which avoided the increase in the series terms at high frequency.As a result,the improved method not only reduces the calculation burden but also ensures the calculation accuracy.Furthermore,in order to guarantee the convergence of the admittance matrix series in the frequency range of interest,the studied transmission line was divided into some small sections.Under the same conditions,the calculating result of the admittance matrix of a line section was compared with the result by Matlab,and the line harmonic current result was compared with the result by PSCAD/EMTDC,both show a good consistency,and this verifies the validity of the improved method.