The invention relates to a modulus correlation-based MMC DC transmission line longitudinal protection method, which belongs to the technical field of electric power system relay protection. When the MMC DC transmission line fails, first collect the positive and negative pole voltages and currents at both ends of the line, and obtain the line-mode voltage and current according to the phase-mode transformation, so as to calculate the correlation coefficient of the line-mode voltage and current at both ends of the line, and then use the correlation coefficient to carry out Identification of internal and external faults, if the judgment result is that one end is positive and the other end is negative, it is an external fault, and if both ends are negative, it is an internal fault. The invention has a strong ability to withstand the transition resistance, the data window is only 3ms, the quick action is good, and the application prospect is strong.