Based on Kirchhoff's law, current differential protection can protect a transmission line with superior selectivity. The influence of the parameter distribution characteristic increases with the length of the transmission line. The differential current protection scheme presently being used for an HVDC transmission line has to increase the time delay and protection threshold to avoid mis-operation induced by distributed capacitor current along the line, which leads to low operational speed and sensitivity. To solve these issues, an improved differential protection criterion using the calculated current at a compensation point of the transmission line is studied. The mechanisms which produce its unbalanced current induced by the coupling characteristic are analyzed and simulated. Based on the distributed parameter model, a current differential protection algorithm using the compensation current of multiple points is presented for HVDC transmission lines. The proposed algorithm is independent of the parameter characteristics of the transmission line and can split-pole operate during the fault transient process with high sensitivity. Its feasibility and validity are verified based on the simulation results and comparative results of a variety of fault scenarios. This work is supported by the National Natural Science Foundation of China (No. 51807053), the Natural Science Foundation of Hunan Province (No. 2020JJ5056), the Fundamental Research Funds for Central Universities (No. 531107051113), and the National Key Research and Development Program of China (No. 2018YFB0904600).