About double circuit transmission high voltage direct current（DCT-HVDC） project， there is no actual operation and design experience in the world. The previous alternating current side harmonic calculation and filter design methods are only suitable for the traditional single circuit HVDC project， and can not be used for the harmonic calculation and filter design of the DCT-HVDC project.In this paper， the technical difficulties of DCT-HVDC are analyzed， and some solutions are proposed. First， based on the characteristics of the HVDC system， the harmonic voltages on the entrance of the DC line are assumed to be zero. It decouples the harmonic current calculation process of the rectifier side and inverter side， and avoids the modeling of the DC line in the harmonic current calculation. The DCT-HVDC project can thus be reduced as two traditional single circuit HVDC projects. Second， two operation modes of the single circuit HVDC projects are synthesized as a new DCT-HVDC operation mode. In this mode， the maximal harmonic combination is picked up among different power combinations as the harmonic current sources for the performance and rating calculation， which eliminates the adverse effect of different power combinations on harmonic calculation and greatly reduces the computation burden. Third， the algorithm and basic principle of filter switching strategy are studied. At last， the validity of the proposed method is demonstrated by the engineering calculation and preliminary design of the Xiluodu-Guangdong±500 kV DCT-HVDC project.