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Accurate and rapid estimation of evapotranspiration in farmland is significant for precise irrigation management and optimal allocation of water resources. Based on 24 cluster lysimeters, irrigation experiments of winter wheat and summer maize were carried out in 2019—2020, in Daxing Water-saving Irrigation Experimental Station of IWHR, Beijing. Daily data of crop canopy temperatures (Tc) and air temperatures (Ta), and daily evapotranspiration measured by lysimeter (ETa) were observed, to calibrate and validate the named S-I model, which was simple and valid to estimate field crops ET. Then the changes and values of feature parameters of S-I model, a and b, were analyzed and concluded for winter wheat and summer maize, respectively. Results showed that for winter wheat, the characteristic parameter a of the S-I model was increased firstly and decreased subsequently with time change in day. In the case of severe water stress, a was negative and the value was small, and in the case of other irrigation treatments, a was positive and gradually changed to negative. The characteristic parameter b in different irrigation treatment was negative. The parameter b of adequate irrigation treatment was gradually increased with time changes in the day, while the value of severe water stress treatment was relatively large, and the trend in day was unstable. For summer maize, the effect of water stress on model parameters was lower than that of winter wheat. The characteristic parameter a was positive, while the parameter b was negative, which was gradually increased with time. The variation range of parameter b under water stress treatment was significantly smaller than that under other two treatments. The characteristic parameters of drought treatment was changed greatly within days. Model parameters a and b varied greatly between winter wheat and summer maize under different treatments, but the fitting accuracy of (canopy temperature Tc-air temperature Ta) and (daily evapotranspiration ETd-daily net radiation Rnd ) was the highest at 13:00. At this time, model parameters a and b of full irrigation were 1.082 and -1.127 for winter wheat, 1.588 and -1.363 for summer maize, respectively. The determination coefficient (R2) and consistency coefficient d were above 0.7 and 0.9 between the model ETd and measured ETa during the growth period of winter wheat and summer maize, while the values of root mean square error (RMSE) were less than 0.89mm/d. As a contrast, the values of R2 and d in the full irrigation treatment were both higher, and the RMSE was lower than that in other treatments, indicating that water stress affected the estimation accuracy of the model, and the S-I model could be better to estimate the crop evapotranspiration under less water stress.