The invention discloses a diagnosis method and system for judging electricity stealing behavior of an electricity saver. The method comprises the following steps that an intelligent electric energy meter collects active power P1O and reactive power Q1O of a first node when a first switch is disconnected; the intelligent electric energy meter collects active power P1C and reactive power Q1C of thefirst node when the first switch is switched on; a high-precision multi-path synchronous data acquisition system acquires active power P2O and reactive power Q2O of a second node when the first switchis disconnected; the high-precision multi-path synchronous data acquisition system acquires active power P2C and reactive power Q2C of the second node when the first switch is switched on; the high-precision multi-path synchronous data acquisition system acquires active power P3O and reactive power Q3O of a third node when the first switch is disconnected; and when the active power P1O, the active power P2O, the active power P3O, the active power P1C and the active power P2C are equal to each other, and the reactive power Q1O, the reactive power Q2O, the reactive power Q3O, the reactive powerQ1C and the reactive power Q2C are equal to each other, and the electricity saver is diagnosed as not saving electricity and not stealing electricity.