The invention discloses a method and a system for partitioning total output of a single shaft gas-steam combined cycle generating set. The method comprises the following steps: (a) establishing a mathematical model of a gas compressor, a combustion chamber and a gas turbine; (b) acquiring a real-time production parameter of the generating set; (c) calculating the power of the gas turbine and the consumed power of the gas compressor according to the results of the steps (a) and (b); (d) deducting the consumed power of the gas compressor from the generated power of the gas turbine to obtain theoutput of the gas turbine; (e) listing a material balance equation and an energy balance equation for a waste heat boiler; and (f) establishing a mathematical model of a steam turbine, listing a heatbalance equation for the waste heat boiler, and calculating the output of the steam turbine through the mathematical model of the steam turbine. The method and the system can master the contributionsof the gas turbine and the steam turbine to the total output of the set and the performance of each key part in real time so as to perform effective adjustment and control according to the actual conditions of the set.