This paper firstly introduces the influence of the voltage quality when it is less than lower limit, which is caused by a weak point of the distribution network. And it is urgent to invent a method to find weak points of the distribution network. Secondly, it establishes resistance equivalent model in three-phase power system, a resistance equivalent model with a weak point existed, and introduces the concept and calculation formulas of voltage difference between two measurement points, resistance value and resistance comparison value. And a device, including measuring transducers of electrical volume and showing results with an intelligent display, is applied to find the weak point of 400V distribution network. Thirdly, an experiment, designed by a weak point of distribution network simulated by the resistance element, a power line with daily load simulated by an electrical cable with an electric kettle, was performed in the laboratory. It proves that this device, based on the voltage difference between two measurement points reached the criterion of low voltage, comparison between measured resistance value and resistance comparison value, is able to find whether there are weak points along the line. And the result shows that it can locate the weak point of distribution line weaknesses in perspective. It has greater help to find the weak point of distribution network, and product demand is so continuous and large that the product's market is broad.