Voltage　sag　is　one　of　the　major　power　quality　problems.　As　far　as　we　know,　it＇s　difficult　to　determine　which　side　of　the　monitoring　point　the　voltage　sag　source　originates　just　using　the　voltage　sag　signal.　This　paper　proposes　a　new　method　for　voltage　sag　source　location.　This　method　uses　the　RMS　voltage　to　decide　whether　the　voltage　sag　originates　and　adopts　the　ratio　of　voltage　to　current　(measured　impedance)　to　determine　the　voltage　sag　source　location.　The　efficiency　of　the　method　is　verified　by　MATLAB/SIMULINK　under　the　loads　of　power　system　are　the　constant　impedance　and　the　asynchronous　motor　respectively.