HVDC transmission Open Line Test (OLT) is the normal operation of an important means to detect the converter valves, DC equipment, and DC transmission line insulation. In this paper, the equivalent circuit for quantitative calculation of HVDC OLT is established based on the theory of power electronics and circuit theory and combined with the actual operating parameters and trigger control mode of converter valve in HVDC project. This paper analyzes and calculates the test principle with and without DC line OLT respectively and the process of establishing the open line voltage, and explains the principle and physical essence of establishing open line voltage under the two conditions. On this basis, the equivalent circuit is used to calculate the voltage of OLT under different conditions. The calculation results are more accurate than the traditional formula. The CIGRE simulation model of HVDC system is built by using PSCAD/EMTDC simulation software, and the simulation results are analyzed and verified. This work is supported by National Natural Science Foundation of China (No. 51777079), Smart Grid Joint Fund of State Grid Corporation of China (No. U1766213), and Science and Technology Project of China Southern Power Grid (No. ZBKJXM20180104).