With the increasing application of synchronverter technology in renewable energy power generation, power system stability issue becomes more complicated. Due to the impedance interaction between the synchronverter based wind farm and the series compensated transmission line, the synchronverter based wind farm is prone to cause sub-synchronous oscillation (SSO). To suppress this SSO, a generalized harmonic compensation control strategy of active power filter (APF) with supercapacitor is proposed, which counteracts SSO by injecting the generalized harmonic current including reactive and oscillation current. Since the generalized harmonic current can be calculated by subtracting the sinusoidal active current from the sampling current, a first-order inertial controller of the voltage loop for APF is proposed to accurately obtain the active current. In addition, the supercapacitor of APF is used to reduce the dc-side voltage fluctuation caused by SSO. As this paper does not need to directly extract the SSO component, the proposed method does not need to know the system oscillation frequency in advance, which can be used as a defensive control method of SSO. Finally, experimental results verify the effectiveness of the proposed strategy.