The harmonic detection method based on FBD power theory was widely used in active power filters because of its simple implementation and strong stability. However, the detection accuracy of traditional FBD method is reduced due to the phase-locked loop unlocking under the voltage distortion of the power grid, and there is inherent contradiction between the steady-state error and the response speed when using the low-pass filter.To solve this problem, an improved method is proposed. In this method, the positive sequence voltage component is extracted quickly by shifting the three-phase voltage by 600 to replace the reference voltage signal to participate in the operation, so as to avoid the detection error caused by voltage asymmetry.An adaptive filter with variable step size based on hyperbolic tangent function was constructed to obtain the equivalent active conductance of fundamental wave. The adaptive algorithm and FBD detection algorithm were integrated organically, and the dynamic response speed of the algorithm was improved effectively.Theoretical derivation and simulation results confirm the correctness and effectiveness of the improved algorithm.