The invention relates to the technical field of power distribution network fault detection, in particular to a power distribution network fault line selection method combining power frequency and transient state reactive power directions. After a single-phase earth fault occurs, an outgoing line with a large transient state zero sequence current amplitude value is firstly selected as a fault alternative outgoing line; then the transient state reactive power and power frequency reactive power of the fault alternative outgoing line are calculated, a sign function of the power frequency reactive power is utilized for correcting the transient state reactive power, and the corrected transient state reactive power is utilized for finding out a fault outgoing line. The problem of single-phase earth fault line selection with uncertain TV and TA polarity in a small current neutral grounding mode power distribution system is solved. During practical application, the TV and TA polarity of a power distribution network is unknown, the problem of single-phase earth fault line selection with uncertain TV and TA polarity in a small current grounding system is solved, and wide practical application value is achieved.