The invention relates to a longitudinal differential protection method of a power transmission line. The method comprises the steps of: collecting input and output signals in a power transmission line; acquiring sequence components of electrical signals at an input end; calculating sequence components of electrical signals at an output end so as to acquire a function expression of three-phase electrical signals at the output end of the power transmission line and calculate the values of electrical signals in each phase at each time; comparing the values with collected values; and accurately determining whether to carry out longitudinal differential protection according to an error value. The invention has the advantages of easy operation and accurate calculation; the relative error can be completely controlled within 1% according to actual demands; the method is free from the effects of system operating methods, network structures and fluctuations of the grid frequency; the compensation to distributed capacitance current is not needed, and the difficult problem that the accuracy of a line model is affected by the distributed capacitance current is solved; because the calculated object is an instantaneous value and is a scalar, the calculation time is shortened obviously, and the longitudinal differential protection time can be completely controlled within 5ms, and thus the purpose of quickly eliminating faults is achieved, and the security and the stability of the system are improved obviously.