Aiming at the deficiencies of traditional detection methods under distorted power grid voltage and fluctuant frequency, an improved Root-Mean-Square (RMS) harmonic detection method without Phase Locked Loop (PLL) is proposed. This method uses the fundamental positive sequence extraction segments instead of phase locked loop, so as to obtain the signals synchronized with the fundamental positive sequence component and then detect the harmonic component. Compared with the traditional ip-iq detection method, this method can be used effectively at severe condition of power grid with fast response speed. Simulation and experimental results verify that the compensation of power grid side harmonic can achieve the desired results when the method is used in the active power filter. This work is supported by the Specialized Research Fund for Doctoral Program of Higher Education of China (No. 20110095110014) and Key Grant Project of Chinese Ministry of Education (No. 311021).