Abstract: Electricity theft is a global issue, not only causing waste of power resources but also threatening the operational safety of the power grid. This paper focuses on the study of three-phase illegal electricity theft from electric energy meters. Firstly, it delves into the mechanism of energy measurement in electric energy meters, classifies different types of electricity theft, and calculates the power changes they cause respectively. Furthermore, it simulates and calculates the ratios of measured power to actual power after adopting various electricity theft methods, revealing the benefit of each theft behavior. The research in this paper will help power companies and related enterprises to monitor and prevent electricity theft more accurately.