A reactive power compensator device includes an AC capacitor and a small-capacity power converter serially connected thereto that avoid the problems of power resonance and harmonic current injection. The reactive power compensator device provides with compensation reactive power current with fixed amplitude or adjustable amplitude. The power converter permits one of power lines of a power system to be connected to either a positive or negative dc terminal of the power electronic switch set through no power electronic switch. Consequently, the number of the power electronic switches is reduced.