The invention discloses a building electrical energy saving early warning system in the building energy saving field. Through a sampler, an electricity value of each low voltage outgoing line is acquired. And then a subtraction arithmetic unit, a first accumulator and a second accumulator are used to calculate and acquire an outgoing line loss value delta Pa(t) under a buscouple section switch segment model and an outgoing line loss value delta Pb(t) under a buscouple section switch paralleling model, and determine a state of a buscouple section switch. If the buscouple section switch is in a segment mode at outgoing line time t, an early warning signal is emitted when the delta Pa(t) is greater than the delta Pb(t). If the buscouple section switch is in a paralleling mode at the outgoing line time t, the early warning signal is emitted when the delta Pa(t) is less than the delta Pb(t). The system has advantages that an acquisition amount of data is reduced; a response speed is accelerated; the buscouple section switch is reasonably controlled; low voltage losses of a building can be reduced; an electric power resource utilization rate is increased and a purpose of user energy saving is reached.