The invention provides a gas-steam combined system and an operation control method thereof. The gas-steam combined system comprises a gas generator set, a steam circulating loop and a pre-heating heat exchanger, wherein the pre-heating heat exchanger is provided with a first heat exchange channel and a second heat exchange channel which are matched with each other; a waste heat flue is in butt joint with a flue gas outlet of a gas turbine engine; the inlet of the first heat exchange channel is in butt joint with a pre-heating water pumping hole; the outlet of the first heat exchange channel is in butt joint with a pre-heating water compensating hole; the inlet of a second heat exchange channel is in butt joint with a fuel source or an air source; and the outlet of the second heat exchange channel is in butt joint with a fuel inlet or an air inlet of the gas turbine engine. By virtue of the pre-heating heat exchanger, the fuel or air temperature is increased, so that the exhaust gas temperature after the gas turbine engine does work is increased, steam with greater flow enters the gas turbine engine to do work, and therefore, the efficiency of the whole gas-steam combined system is improved, the flue gas exhaust temperature is reduced, and the thermal efficiency of the whole gas-steam circulation is improved.