The invention discloses a vacuum cleaner, a control method, a control device and a control system for a brushless direct current (DC) motor, wherein the method comprises the following steps: obtaininga temperature-phase resistance meter, a temperature-phase inductance meter and a bus voltage-phase current change rate table offline; in the low-speed operation phase of the brushless DC motor, acquiring a conducting phase positive terminal voltage, a conducting phase negative terminal voltage, a non-conducting phase opposite potential voltage, a bus current instantaneous value, a bus voltage, and a current winding temperature during the high-level period of each PWM control cycle; acquiring a phase current change rate according to the bus voltage and the bus voltage-phase current change ratetable, and acquiring a phase resistance and a phase inductance according to the current winding temperature, the temperature-phase resistance meter and the temperature-phase inductance meter; acquiring a flux linkage value of the brushless DC motor according to the above parameters, and determining whether the flux linkage value is greater than the commutation threshold; and if so, performing commutation. Therefore, the accurate commutation of the motor may be realized in the low speed running phase of the motor, the motor may be prevented from being out of step, and the reliability is high.