The invention provides a building stand-by power supply power generation system used for supplying power to an external load. The stand-by power supply power generation system comprises a plurality ofgenerating units and a control cabinet connected to the generating units, the control cabinet is used for controlling the total output power of the generating units, the control cabinet comprises a parallel-operation control unit, the parallel-operation control unit can correspondingly adjust the quantity of the generating units in the operating states and the output power of each generating unitin the operating state according to the change of the power required by the external load, and the power required by the operation of the external load and the quantity of the generating units in theoperating states satisfy the following condition: when 0<=P<=4/5*M*A, the formula P<=4/5*N*A is satisfied. The building stand-by power supply power generation system can adapt to large-amplitude instantaneous changes of the power in a building site, and the condition of failure of timely supply due to large increase of the instantaneous power is avoided.