The calculation of main circuit parameters plays a critical role in the design of HVDC transmission system and contributes a great practical significance to the further development of HVDC projects. Firstly, this paper describes the purpose of main circuit parameters calculation, and introduces the basic conditions and data of HVDC transmission system that are necessary in the calculation and design of main circuit parameters, based on the basic theory of DC transmission and the characteristics of HVDC system. Then, according to these factors, the provided raw data and the basic principle of DC transmission, the basic parameters and operation characteristics of primary equipments were calculated such as converter transformer and converter valve. Finally, the correctness tests of the computing platform function was carried out on an actual HVDC transmission project.