The invention discloses a method for applying a phase-fault overload protection device of a motor with an isolating switch motor driven operating mechanism, which comprises the following steps that: a secondary electric control system of the operating mechanism is divided into two parts, namely a motor circuit and a control circuit; lines A, B and C of the motor circuit pass through three mutual inductor holes of the phase-fault overload protection device respectively, and the symmetry degree of current is transmitted to the phase-fault overload protection device through the induction of a mutual inductor for the motor circuit; and lines D and E of the control circuit are connected to two connection points of the phase-fault overload protection device, and the phase-fault overload protection device controls current of the control circuit. If the current is asymmetrical, namely the current lacks phase and the current exceeds a normal range, the phase-fault overload protection device can automatically cut off a power supply to eliminate potential safety hazard.