The power, voltage and current will change along with the change of grid’s operation mode; however, the line resistance is unchanged. This paper analyzed the limitation and inadaptability of the existed methods used for anti-electricity-theft. And most of the criterions are based on the variable electrical quantities. In order to establish a criterion that can find out the position of electricity-theft accurately, it chose line resistance as a reference value, and calculated the equivalent resistances of each node and branch combining with the actual measurement system. To verify the validity of this criterion, a medium and low voltage distribution network was structured in the Matlab/Simulink simulation platform. The results, which are basically consistent with the actual situations, show that the criterion based on line resistance can well investigate electricity-theft, and also verify that the introduction of this method will bring great benefits to engineering applications.