The invention discloses a system and a method for diagnosing electricity stealing on the basis of decision tree algorithms. The system comprises a data interface module, an electricity stealing diagnosis decision tree module, a diagnosis result output module, storage equipment and a man-machine interface, and is characterized in that historical data of users can be extracted from a power grid database, can be transmitted to the data interface module and then can be fed into and the electricity stealing diagnosis decision tree module, and inference results can be formed, can be stored in the storage equipment by the aid of the diagnosis result output module and can be outputted to the main-machine interface. The system and the method have the advantages that the historical data of the users can be analyzed by the aid of the decision tree algorithms, and accordingly electricity stealing users can be detected.