Abstract: Taking a 660 MW supercritical unit as an object of study, two off-design models were set up respectively for the steam turbine and air-cooling system based on its operation data obtained under summer conditions, of which their calculation accuracy was subsequently verified with the performance curves for constant dryness and the data in original calculation specifications. Moreover, the way how to determine the optimal vacuum of the condenser was analyzed under different boundary conditions, including the air temperature, the heat exchange of condenser, the load and the fan speed, etc. Results show that under summer conditions, when the load is relatively high, an overclocked operation should be kept for the fan, so as to guarantee a safe backpressure and operation for the cooling system; whereas when the load is relatively low, in case of a fixed extraction flow, the pursuit of high operation economy becomes important, since the difference between the rise of unit output and the rise of fan power consumption has a maximum value with the increase of cooling air flow.