The invention relates to an indirect air cooling high-back-pressure heat supply machine unit back pressure control system and method. The system comprises a steam turbine low-pressure cylinder, a double-temperature-region steam condenser, an indirect cooling tower and a DCS control system, wherein a variable-frequency circulation water pump and a thermal resistor are arranged on a circulation water pipe entering the side B of the double-temperature-region steam condenser from the indirect cooling tower; the variable-frequency water pump is connected with a frequency converter; a pressure transducer is arranged at the side B of the double-temperature-region steam condenser; the frequency converter, the thermal resistor and the pressure transducer are connected with the DCS system through cables; a PID controller is used in the DCS control system for controlling the frequency converter; the back pressure practical value PV is measured through the pressure transducer; the PID controller outputs signals to the frequency converter used as an executing unit so as to regulate the frequency of the frequency converter; the power of the variable-frequency circulation water pump is further regulated; the flow rate of the circulation water is regulated; the goal of regulating the pressure of the double-temperature-region steam condenser is finally achieved. The system and the method provided by the invention have the advantages that the back pressure set value is optimized; the fuzzy self adaptation PID control is used for improving the dynamic regulating process; the ordinary PID control overshoot is reduced.