The invention relates to a time difference method based adaptive half wavelength line differential protection method. As the electric distance of half wavelength power transmission is long, obvious actuation time difference between protective starting elements at two sides of the line exists after faults occur. According to the wave transmission principle, the position of a fault point can be estimated according to the actuation time difference of the protective starting elements at two sides of the line. Voltage and current of the protective installation positions at two sides of the line can be compensated to the fault point according to a long line equation, so as to obtain a current value of the fault point, and then differential current is calculated. Sensitivities for fault differential protection criterions of different points of the line are different, and brake coefficients and threshold values of differential protection are changed according to different adaptions of the fault point positions, so as to ensure that differential protection has sensitivity for the whole line.