The invention relates to a transmission line time domain full waveform protection device and a phase-to-phase catastrophe quantity direction judging method. By acquiring the first set of interphase current instantaneous values, a second set of interphase current instantaneous values, a first set of interphase voltage instantaneous values, a second set of interphase voltage instantaneous values after the fault is acquired and before the set period, a sudden change amount of phase-to-phase current and a sudden change amount of phase-to-phase voltage are calculated, and then the equations are established according to the constraint relation between the current and voltage of the abrupt change, The magnitude of positive sequence reactance and positive sequence resistance can be obtained by solving the problem that the existing directional elements can not accurately and reliably judge the direction of phase-to-phase fault, and the existing directional elements are not reliable in the powerelectronics power system, so that whether the fault is a positive fault or not can be accurately judged.