For a hybrid high-voltage direct current（HVDC）transmission system using the line commutated converter（LCC）at the rectifier side and the modular multilevel converter（MMC）at the inverter side，a calculation method and the complete process of harmonic current at the DC side are proposed.Firstly，the LCC at the rectifier side is replaced by the three-pulse harmonic voltage source，whose harmonic output characteristic is equivalent to a twelve-pulse converter.Secondly，the MMC is represented by a passive electric network，with an improved calculating method for the coupled line model introduced for calculating the admittance matrix of DC transmission lines.Finally，by comparing with harmonic current calculation results at the DC side of a digital simulation model based on PSCAD/EMTDC，the accuracy of the proposed method is verified.