Based on Penman-Monteith equation, a simplified formula for calculating the crop evapotranspiration in greenhouse was deduced by introducing the parameter of crop canopy height and modifying the item of aerodynamics related to wind velocity. The deductive procedure was analyzed theoretically, and the formula was validated with meteorological data. The results showed that the modified Penman-Monteith equation had a higher precision of prediction, with a relative deviation of 4. 7%-17. 1% and 11. 1% on average, being suitable to calculate the crop evapotranspiration in greenhouse.