A current detection method of improved conservative power theory that effectively decomposed the fundamental active current and eliminated the influence of harmonic wave on grid voltage was presented in this paper. The current detection method used in the one-cycle control of three-phase three-wire active power filter system, the simulation and experimental results validate the correctness of the scheme,that eliminated the influence of the harmonic wave in the power grid, suppressed harmonic current and reactive power of nonlinear load generated.