The invention relates to an online low voltage detection device for electric leakage and electric energy steal and an implement method thereof. The online low voltage detection device for electric leakage and electric energy steal comprises a first current mutual-inductance sampling apparatus, a second current mutual-inductance sampling apparatus, a first analog-digital converter, a second analog-digital converter, a computing controller and a display module; the output end of the first current mutual-inductance sampling apparatus is connected to the first input end of the computing controller through the first analog-digital converter; the output end of the second current mutual-inductance sampling apparatus is connected to the second input end of the computing controller through the second analog-digital converter; the first data output end of the computing controller is connected to the display module. According to the online low voltage detection device for electric leakage and electric energy steal and the implement method thereof, once the first current mutual-inductance sampling apparatus and the second current mutual-inductance sampling apparatus are respectively connected to the input side or the output side of an electric energy meter, whether electric leakage or electric energy steal line exist on the output side of the electric energy meter can be automatically detected, electric leakage and electric energy steal detection to an electric power consumer is achieved without power cut on electric transmission lines, and the difficulty in electric leakage and electric energy steal detection is lowered.