The invention discloses a current second-order gradient-based single-ended data protection method for a high-voltage direct-current transmission line. The method comprises the steps of measuring the current at each end of the line, calculating the second-order gradient integral value of the current, and recognizing troubles inside the sample space through comparing the second-order gradient integral value of the current with a starting threshold. The method is simple in principle, and can be realized only by adopting the whole content of a single-ended current. Meanwhile, the filtering is not needed, and the method is easy in implementation. In viewing from the principle, a most serious external fault is guaranteed. That means, the method is free of maloperation and good in selectively during the external metallic grounding fault of a smoothing reactor. Based on the cooperation of existing fault pole selection elements, the correct pole selection action can be achieved. The method is fast in action speed and realizes the complete setting principle. The full-line high speed can be realized. Compared with traveling wave protection, the method is not influenced by noises and is high in reliability. Compared with break variable protection, the method is not influenced by the most serious external fault and is good in selectively. The method can be used for the main protection of the high-voltage direct-current transmission line.