Abstract: To solve the evaluating problem of the water surface evaporation under the coverage of spheres, the P-T model coefficient α is calculated by the following two methods, based on the weights of changes in the overall surface energy balance components covered by spheres during the complete non-freezing period. (1)Bowen ratio β is introduced to establish the calculation formula of the P-T coefficient α₁; (2)the P-T coefficient α₂ is obtained by the linear fitting method across the origin, which taking the radiation term in the P-T formula as the independent variable, and the latent heat flux corresponding to the measured water surface evaporation as the dependent variable. According to the monthly change trend of Bowen ratio β and coefficient α during the complete non-freezing period, the P-T model applied to the estimation effect of the water surface evaporation under the coverage of spheres was evaluated in the complete non-freezing period, odd months, and even months. The results show that the P-T model can better estimate the evaporation of the water surface covered by spheres, and the simulation effect of the P-T model corresponding to the coefficient α₂ (0.42~0.44) is better than the simulation effect of the P-T model corresponding to the coefficient α₁ (0.39~0.60). The conclusions of this study have reference value for scientific evaluation of evaporation prevention and water saving effect of plain reservoirs in arid areas covered by spheres.