The analysis was conducted for the optimal operation under off-design conditions of the indirect dry cooling thermal power generating unit. The numerical model of natural draft dry cooling tower (NDDCT), as well as the off-design performance model of whole thermal system of a supercritical power generating unit, including that of turbine, condenser and regenerative system, were combined together. An iterative procedure was adopted so that the operating parameters of dry cooling system with cooling water mass flow rate and output power load were obtained. The total coal consumption of power generation, which considered both effects of back pressure of turbine and power consumption of circulating pump, was taken as the optimization object. The results indicated that the pump power consumption increased, while the back pressure decreased with circulating cooling water mass flow rate. There was the optimum cooling water mass flow rate corresponding to the lowest coal consumption of power generation. For the objective power generating unit, 4.83 ton standard coal per hour at most can be saved by adjusting the circulating cooling water flow rate to the optimum value. (C) 2018 Elsevier Ltd. All rights reserved.