The invention discloses a transmission line positive sequence current split-phase differential protection method based on a short data window. The method comprises the following steps that firstly, three-phase current instantaneous values at the two ends of a transmission line at each sampling moment are converted into current instantaneous values at the two ends of the transmission line under alpha beta 0 coordinate axes at each sampling moment, positive sequence current instantaneous values at the two ends of the transmission line under the alpha beta 0 coordinate axes at each sampling moment are worked out, coordinate axis conversion is conducted on the positive sequence current instantaneous values at the two ends of the transmission line under the alpha beta 0 coordinate axes at each sampling moment so as to obtain three-phase positive sequence current instantaneous values at the two ends of the transmission line at each sampling moment, and then the three-phase positive sequence current instantaneous values at the two ends of the transmission line at each sampling moment are used for constituting positive sequence current split-phase differential protection data. According to the method, the algorithm principle is simple, program implementation is easy, the calculation amount is small, calculation speed is high, and protection action speed of a relay can be increased greatly. A phase selection function is achieved, transition resistance and a load current have no influence on action performance, and the method is suitable for achieving a transmission line main protection function in a whole fault process after a fault occurs.