Noise cancellation is used in the traditional adaptive harmonic detection algorithm containing integrator, but the direct signal which is gained from integrating the original signal containing active components will fluctuate. In order to handle this issue, this paper proposes an adaptive harmonic detection algorithm based on Fourier series. Applying the decomposition principle of Fourier series, the adaptive algorithm is used to correct the coefficients of every order’s Fourier series, and then the fundamental active and reactive current amplitudes are gained. The fundamental active and reactive current can be calculated by multiplying the fundamental active and reactive current amplitudes respectively with the unit sin and cos signals which are output by PLL. The simulation proves that the method proposed in this paper can fast detect the fundamental active and reactive current amplitudes steady and depress the ripple of DC signal.