Aiming to facilitate irrigation management of crops grown in greenhouse, a model was developed to estimate the reference evapotranspiration (ETo) inside greenhouse during ventilation by using outdoor meteorological data. An experimental study was carried out in May—June 2020, 2021 under grass cultivation conditions in a plastic greenhouse. The Penman-Monteith (PM) equation was used to calculate ETo inside and outside the plastic greenhouse, and variations of the ETo radiative component (ETo,rad) and the ETo advection component (ETo,adv) inside and outside were also analyzed. Thereafter, the radiative transmittance τ and wind speed attenuation rate ω were introduced to improve the radiative and advection sub-equations of the ETo equation, then a simple model for estimating indoor ETo was constructed based on outdoor meteorological data. The results showed that the average ETo, ETo,rad, and ETo, adv of indoors were decreased by 45.6%, 17.1%, and 94.0%, respectively, compared with that of outdoors. During the experiment, the attenuation factors of radiation components ζrad, advection components ζadv, and total flux ζo showed a tendency of ζrad >ζo > ζadv. The two years average ζradwas 23.1% to 33.3% and 90.1% to 93.3% higher than that of ζo and ζadv, respectively. Furthermore,a highly significant linear relationship (P<0.01) between ζrad and τ, as well as between ζadv and ω was found, with R2 of 0.74 and 0.90, MAE of 0.07 and 0.04, RMSE of 0.09 and 0.05, respectively. The simple model equation for estimating indoor ETo based on outdoor meteorological information included four meteorological parameters, including net radiation, air temperature, water vapour pressure deficit, and wind speed, moreover, two greenhouse characteristic factors,τand ω were also considered. The developed model was robust by comparing with the measurements, with R2 of 0.96, MAE and RMSE of 0.17mm/d and 0.22mm/d, respectively. This model developed had high precision and few parameters, and was easy to obtain, which can provide a reference method for the facilitation of greenhouse group irrigation management.