Abstract: In recent years, long-term power failure accidents of submarine DC cables have occurred frequently due to damage factors such as seawater erosion and anchor damage, posing a serious threat to the safe and stable operation of underwater transmission systems. It is of great significance to locate the fault source quickly and accurately for improving the reliability of cable insulation. However, the existing spread spectrum time-domain reflection method has low testing efficiency and accuracy due to its limited ranging range constrained by symbol duration. Therefore, this paper proposes a mixed modulation method, which multiplies m sequences with different symbol durations and modulates them through binary phase shift keying using a sine signal, resulting in a mixed frequency signal rich in fault information. This method can realize wide range ranging and improve the testing efficiency and accuracy. Through model derivation and experimental verification, a submarine cable break fault is simulated and studied for location. The results show that the period of the m sequence is proportional to the ranging range, while the duration of the symbol is positively correlated with the blind spot of the ranging. Based on this, modulation experiments are carried out on m sequences with symbol durations of 1μs and 0.1 μs. The results show that the ranging range is extended from 3 141 m and 314.1 m to 3 235.23 m, respectively, and the ranging blind spot is further reduced. Therefore, using the mixed modulation method proposed in this paper can effectively overcome the drawbacks of existing methods and greatly improve the efficiency and accuracy of submarine cable fault location.