Ultra High Voltage Direct Current (UHVDC) transmission lines usually go across long distance, and hence are exposed to high fault probabilities. Its conventional current differential protection lacks rapidity, as well as adaptability to high fault resistances. The correlation coefficient can be used to quantitatively describe the trend of two variables without being influenced by the magnitude of variations. Accordingly, based on the fault transient characteristic of DC lines, characteristic quantities for relay criteria are generated first by calculating correlation coefficients of the sum and difference of both-terminal currents. Subsequently, considering different characteristics in terms of internal and external faults reflected by relay criteria and the effect of control systems, a new pilot protection principle is proposed. The principle, without compensation requirement of distributed capacitance current, has better capability to discriminate UHVDC line faults with high fault resistance, and faster reaction than current differential protection. It also has lower requirements on communication channels, which means easier to be applied. Finally, the effectiveness of the proposed protection is verified by PSCAD/EMTDC simulation. This work is supported by National Natural Science Foundation of China (No. 51877084), Natural Science Foundation of Hebei Province (No. E2018502063), and Fundamental Research Funds for the Central Universities (No. 2017MS096).