Based on the Merkel method, a mathematical model for calculating the evaporation loss of natural draft counterflow wet cooling towers (NDWCTs) was established. Taking the NDWCTs of the representative 300 MW and 600 MW power plants as the experimental objects, the calculated values obtained by the mathematical method were in good agreement with field experiment. Then, the effect of thermal load on evaporation loss of NDWCT was analyzed. The results showed that with the thermal load of NDWCT increasing, the evaporation loss increased, while the rate of evaporation loss caused by per unit change in temperature decreased. When the thermal load is the same, the evaporation loss was basically equal no matter by changing the mass flowrate or the inlet water temperature.