A new current detecting algorithm is proposed in the paper, which is applied in an improved hybrid active power filter. The filter equipment can be applied in high voltage system to suppress harmonics and compensate reactive power. The topology and filtering principle of the filter equipment is given. The harmonic suppressing function is adopted to analyze the influence by the grid impedance, quality factor and losing resonance of the resonance circuit. The feasibility of the proposed system is validated in theory. A single-phase current detecting method is also proposed. The corresponding current control is formed combined the DC-link current detecting method. The simulating results prove that the system is feasibility and the current control is validity. Harmonic and reactive power are suppressed well.