The invention discloses a theft monitoring method and a monitor by the method for low-voltage electrical installations such as cables, transformers and the like, and belongs to the technical field of antitheft. The electrical installation theft monitoring method comprises the following steps: measuring relative resistance between a zero line and an earth wire; and judging the size of the resistance, generating theft alarm information if the resistance is in the resistance range under the condition of open circuit of the zero line, otherwise not alarming. The electrical installation theft monitor comprises a signal acquisition module which is provided with a zero line interface and an earth wire interface and is used for measuring zero line resistance to the earth to generate on/off state signals of the zero line. The method is simple, has high operability and high accuracy of 100 percent, and can realize low cost.