Conventional DQ transformation method and the PLL technology can detect the amplitude and phase of voltage sag accurately, but the method is only applicable to the case of symmetrical three phase voltage. When three phase voltage unbalance, there is a certain bias in the results by use of this detection method. A new detection method of improved PLL based on dual DQ decoupling transformation is proposed in this paper, it removed the coupling relationship of output voltages by setting the positive and negative sequence voltages reference, so this method can accurately detect the amplitude and phase information of the voltage sag, simulation results validates the proposed method is effective and feasible.