The invention relates to an energy supply system mainly through gas and steam combined cycle cogeneration, and belongs to the technical field of energy sources. The system adopts a mode of combined heating of a smoke condensing heat exchanger, a condenser, a steam absorption heat pump, a high-temperature smoke heat exchanger and a steam-water heat exchanger on a heat source side; return water of a heat supply network directly recovers low-temperature smoke condensed waste heat, high-temperature smoke waste heat or condensed steam waste heat; and simultaneously, the steam absorption heat pump driven by extracted steam is used for recovering condensed steam discharged heat or partial smoke waste heat. During heat supply in winter, a client uses a supplementary-fired hot water absorption heat pump and a water-water heat exchanger to heat secondary water in the heat supply network for heat supply and extract low-grade heat for heat supply from a superficial layer soil source, an underground water source or a geothermal water source; and in summer, the client uses the hot water absorption heat pump to refrigerate hot water in the heat supply network and a liquid desiccant air conditioner to remove humidify and preheat domestic hot water. Compared with the conventional system, the system of the invention has good economy and application prospect; and the energy utilization rate of the system is greatly improved.