An active power conditioner, connecting with a load and a power source in parallel, includes a DC power capacitor, a power converter, a high-frequency filter and a control circuit. The control circuit detects the voltage of the DC power capacitor and the current of the power source, thereby performing a closed loop control to stabilize the voltage of the DC power capacitor and performing a feed forward control to generate a driving signal for driving the power converter. The power converter actuates the DC power capacitor to supply a current via the high-frequency filter to reduce the reactive and harmonic components of load current. The load is converted to the linear resistance characteristic, observed from the power source side.