The present invention discloses a direction judgment method of a voltage sag source based on a sequence space vector characteristic impedance real part polarity. Firstly, an active current reference direction is 'determined' at a monitoring point, then a linear active single-port network theory is applied, and the direction of a voltage sag generation source is accurately judged through measuring the real part polarity of the sequence space vector characteristic of an active single-port network. When the real part polarity of the sequence space vector characteristic is positive, a disturbance source is at the upstream of the monitoring point. When the real part polarity of the sequence space vector characteristic is negative, the disturbance source is at the downstream of the monitoring point. The invention provides the voltage sag source direction judgment method, the generation sources of voltage sag caused by various grid faults can be judged definitively, and the method is suitable for any grid structure grid and neutral point effective and non-effective grounding grids and is also suitable for the positioning of the disturbance source.