A reactive power, unbalanced and harmonic detection method based on SDFT algorithm is as follows: three-phase alternating current signals are sampled at a fixed sampling period to obtain discrete sampling data; a zero sequence component is obtained, and meanwhile a result of removing the zero sequence component is obtained; SDFT conversion is carried out on the sampling data, and the integral result of each current flow under the fundamental wave period point is obtained; according to the calculation formula of DFT, the integral value of each current flow is corrected, and the result of the SDFT operation is replaced; a fundamental wave reactive power and unbalanced component is obtained according to a calculation formula in combination with a power grid voltage phase locking result, and the instantaneous value of the harmonic component can be obtained according to the phase relation; the rotation factor is obtained through a look-up method, only a sine value table is stored, and the memory usage can be reduced; and the stability problem caused by iteration accumulation errors can be eliminated.