The invention relates to a circulating water heat supply system with low-vacuum-running induced draft fan turbines and an energy saving method of the circulating water heat supply system. The circulating water heat supply system with the low-vacuum-running induced draft fan turbines comprises an induced draft fan turbine unit, a cooling tower and a heating user, wherein the induced draft fan turbine unit comprises a plurality of induced draft fan turbine structures; circulating water outlet ends of the induced draft fan turbine structures are connected with an input end of a change-over valve; a first output end of the change-over valve is connected with a circulating water inlet end of the heating user through a heating network circulating water pump structure and a heating network heater structure successively; a circulating water returning end of the heating user is connected with circulating water inlet ends of the induced draft fan turbine structures through a first switch valve to form a heating circulating water loop; a second output end of the change-over valve is connected with a circulating water inlet end of the cooling tower; a circulating water returning end of the cooling tower is connected with the circulating water inlet ends of the induced draft fan turbine structures through a second switch valve to form a non-heating circulating water loop; and each induced draft fan turbine structure consists of an induced draft fan turbine and a condenser which are connected with each other. The efficiency of the induced draft fan turbines and the response rate of variable-load working condition adjustment can be improved, and efficient comprehensive utilization of energy is realized.