High-temperature superconducting direct current (HTS DC) power transmission,having the great merits of low loss,high capacity,environment friendly and etc.,recently attracts more and more attention around the world.Based on the wide investigation of the HTS DC projects,we find that all the existing structures of HTS DC cable are different from each other.So it is necessary to analyze to clarify which structure is the optimal one for long-distance high-capacity power transmission.In this paper,we propose a method of classification and comparison.