The invention discloses an ESO-based pure electric vehicle bus capacitor discharging system and method. The system mainly comprises a controller, a power battery, a circuit breaker, a DC / DC converter, a bus capacitor, an inverter and a three-phase permanent magnet synchronous motor. According to the method, a permanent magnet synchronous motor winding is used as an energy bleeder. According to the method, a discharge process is divided into three stages: firstly, a fixed current is applied to a d axis in a weak magnetic voltage reduction stage, so that a bus voltage is rapidly reduced to a safe voltage, and the system enters a constant voltage stage; in the constant-voltage stage, ESO is adopted to observe the total loss in the system discharge stage, feedforward compensation is carried out, the bus voltage is stabilized at the safe voltage, and when the rotating speed is reduced to be lower than the safe rotating speed, the free deceleration stage is started, and d-axis and q-axis currents are gradually reduced to 0; through the above three stages, safe and rapid discharge of the bus capacitor of a pure electric vehicle driving system in emergency is realized, the surge phenomenon of the bus voltage is avoided, and the method has the advantages of high robustness and high safety.