Abstract: Aiming at the problem that the protection of hybrid three-terminal DC transmission lines is difficult to take into account both speed and selectivity, a non-unit pilot protection based on random matrix theory is proposed. It is found that the directions of voltage and current break variables at each measuring point of the mixed three-terminal DC lines are different between internal and external faults. Firstly, the random matrix is formed by copying and translating the voltage and current break variables at each measuring point, and the eigenvalues of each matrix are calculated. Secondly, the internal and external fault identification and fault section location are carried out by combining the ring law and eigenvalue statistics. Finally, the ratio of polar wave energy in fault section is calculated to realize fault pole selection. Based on real-time digital simulator（RTDS）, a hybrid three-terminal DC system is built for test and analysis. The test results show that the proposed protection criterion is simple and reliable, has strong resistance to high resistance, can quickly and accurately identify internal and external faults, fault sections and fault poles under different fault conditions, does not need clock synchronous, and has low requirements for communication channels.