Abstract: The systems of soil plant environment can be controlled in greenhouse with the water heating migration mode, which is different from open field. In the paper,the crop canopy degrees are introduced and the air dynamics value is modified in Penman Monteith(P-M)equation. As a result,a modified P-M equation that can simplify the calculation of the reference crop evapotranspiration in greenhouse is obtained.The equation is analyzed theoretically and verified by the meteorological data.The results show that the daily evapotranspiration varies with the same trend as the climate factors,but the difference is not negligible. Its precision is higher and the result of the experiment is in accordance with the calculation value through the modified P-M equation.The results show a higher precision and with a daily relative deviation of the prediction 9.1% on average.