The embodiment of the invention discloses a method for positioning a voltage sag source, which comprises the following steps: according to a power network structure, a fault type, a triggering condition of an electric energy quality monitor and a preset constraint condition, calculating to generate a node position for installing the electric energy quality monitor; establishing a voltage sag source identification model; according to a three-phase voltage of a node at which the electric energy quality monitor is positioned, calculating an unbalanced value of the node, and according to the unbalanced value, judging a circuit fault type; according to the circuit fault type, the voltage sag source identification model corresponding to the circuit fault type and a voltage variable quantity measured by the electric energy quality monitor, calculating to generate the similarity; and comparing the similarity and positioning the voltage sag source. The embodiment of the invention also provides a device for positioning the voltage sag source. By the method and the device for positioning the voltage sag source, which are disclosed by the embodiment of the invention, a special fault recorder does not need to be used, so that the cost is saved; and moreover, complex data preparation does not need to be carried out, data can be captured only by an electric energy quality monitoring network and a further data mining technology is easy to use.