To enhance the speed and reliability of low-voltage protection for DC transmission lines in hybrid multi-terminal HVDC systems, a novel low-voltage protection strategy based on an improved cumulative sum (CUSUM) algorithm is proposed. When faults occur at different locations in a ±800 kV hybrid multi-terminal HVDC system, the electrical quantities measured at the protection installation exhibit certain differences. Traditional low-voltage protection lacks a setting basis and has the risk of maloperation. This paper introduces the CUSUM algorithm to extract the characteristics of electrical quantity differences during different faults, thereby ensuring the reliability of protection. At the same time, to improve the speed of protection, the CUSUM algorithm window is improved with fractal theory, making the window adaptive and enhancing the speed of protection. A hybrid multi-terminal HVDC model is established using PSCAD/EMTDC, and the proposed new protection strategy is verified with MATLAB. Simulation results show that the proposed scheme can operate quickly and reliably, with good speed and reliability; it can withstand higher transition resistance and has good tolerance to transition resistance; the criterion relies on single-ended electrical quantities, avoiding interference from noise and data anomalies in communication, and can quickly identify internal and external faults and operate reliably.