Locating the voltage sag source quickly and accurately is of great significance to improve the reliability of power supply and clarify the responsibility of both supply and demand. In this paper,a precise location method of voltage sag source in radial distribution network is proposed based on micro-synchronous phasor measurement unit (μPMU) and binary search method. Firstly,the sag source current is calculated by the layered circuit equipped with μPMU. In each layered circuit,it is assumed that the sag source current is injected into each bus node. The virtual voltage variation at the terminal bus is calculated,and the error is obtained by comparing it with the measured voltage variation at the terminal bus. According to the size of the error,the sag source adjacent bus is determined. Secondly,the virtual bus is set at the midpoint of the adjacent section of the sag source adjacent bus. The binary search method is used to quickly narrow the locating interval and achieve precise locating of the sag source. Finally,the IEEE 33-node model is used to verify the proposed method in MATLAB. The results show that the proposed method has good locating accuracy for different positions or different types of voltage sag sources in radial distribution networks. The proposed method has certain anti-interference ability.