The transformation of the instantaneous value symmetry component applied， the positive sequence space vector of fault component of the voltage and current can be derived.This paper deduces the determinant criterion of the fault direction based on the positive sequence space vector.The algorithm is proved to determinate the fault direction in short data window by theory analysis and simulation experiment，then to determinate to be internal fault or not.The algorithm can shorten the time to judge internal fault or external fault greatly，and can enhance the velocity of clearing up all the internal fault assisted by fast inrush criterion.