Firstly in the paper， system basic conditions and data required for calculating main circuit parameters of high voltage direct current（HVDC） transmission systems are described. Secondly， the idea of selecting the short circuit impedance and on-load tap changer range for converter transformer is discussed in detail. For the different control strategies adopted in actual HVDC transmission projects， the corresponding computational methods of main circuit parameters are discussed. Computational software with autonomous intellectual property for the main circuit parameters has been developed. Finally， the tests are performed on three practical HVDC transmission projects. Compared with the results from using Siemens' and ABB's software， the functions of the software developed are validated to be correct. It is proposed to be used in the practical design of HVDC transmission in future.