This　paper　presents　an　improved　harmonics　detecting　method　based　on　theory　of　instantaneous　reactive　power.　This　improved　method　substitutes　traditional　low-pass　filter　(LPF)　by　integrator,　delay　and　gain,　which　can　reduce　delay　time　of　the　harmonics　detecting　to　1/6　of　source　period,　and　it　is　implemented　in　software　using　DSP　chip.　The　simulation　and　experimental　results　indicate　that　the　improved　method　has　characteristic　of　fast　dynamic　response,　it　can　meet　the　precision　and　the　good　real　time　of　harmonics　detection,　also　it　can　be　realized　easily.