DC transmission technology is developing rapidly, but its line protection still has some shortcomings. As a backup protection, current differential protection is susceptible to the influence of line distributed capacitance and has a long delay. In order to solve the long delay problem of DC line backup protection, this paper proposes a new principle of pilot protection based on low-frequency fault current F-test. The principle is based on the difference in the change trend of the fault current at both ends of the line at the time of the fault. The S-transform is used to extract the low-frequency fault current, and the F-test is used to perform the trend test to identify the internal and external faults. Finally, the double-terminal DC transmission system model experiment proves the correctness of the protection principle and verifies that the principle has strong resistance to transition resistance, distributed capacitance and noise interference.