According to the system requirements of multi-terminal upgrading and reconstruction of LCC HVDC power transmission system，this paper studies the topology structure and power transmission mode of multi-terminal HVDC transmission system. A reconstruction method based on the application of DC high-speed switch （HSS） in the intermediate converter station is proposed，and the basic ideas and specific measures of DC field transformation project under extremely site constraints are discussed. In addition，this paper demonstrates the advantages of three-dimensional design in converter station engineering in multi-terminal reconstruction project. This research is based on the first three-terminal transformation HVDC transmission project in China to use the original LCC HVDC transmission channel，and its system design and engineering practice are of typical significance.