The invention discloses a neutral line disconnection detection method based on third-harmonic currents. The neutral line disconnection detection method comprises the following steps: step 1, the decrease rate delta I 3% (absolute value) of the third-harmonic currents is measured simultaneously on three phase lines of a power supply output end, and if the delta I 3% (absolute value) is larger than or equal to a first threshold value sigma, a fault happens; step 2, if the decrease rate delta I 3% (absolute value) of the third-harmonic currents is smaller than the first threshold value sigma and larger than or equal to a second threshold value beta, and the change rate K of fundamental currents on the three phase lines is larger than or equal to a third threshold value mu, a fault happens; step 3, if the decrease rate delta I 3% (absolute value) of the third-harmonic currents is smaller than the second threshold value beta, a fault does not happen; step 4, if the decrease rate delta I 3% (absolute value) of the third-harmonic currents is smaller than the first threshold value sigma and larger than or equal to the second threshold value beta, and the change rate K of the fundamental currents on the three phase lines is smaller than the third threshold value mu, a fault does not happen. The neutral line disconnection detection method based on the third-harmonic currents has the advantages that the detection of a neutral line disconnection fault at the power supply output end is realized, the number of neutral line disconnection detection equipment is reduced, and investment cost of power supply lines is lowered.