Using“Jingxiu”watermelon as the test material，based on the weighing lysimeter，the evapotranspiration law of the autumn mini －watermelon in the greenhouse and its correlation with the environmental factors were studied． The aim was to provide the basis for precision irrigation of greenhouse autumn watermelon． The results show that the evapotranspiration of mini－watermelon is 114．79 mm in the whole growth period，and the average daily evapotranspiration is 1．11 mm/d． The autumn mini－watermelon in the greenhouse shows a small change in the early stage，medium to large stage，and late stage． Among them，the daily evapotranspiration at the seedling stage，vine extension stage，flowering fruit setting stage，fruit expansion stage and maturity stage are 1．04，1．20，1．34，1．08，0．81 mm/d，respectively． The environmental factors significantly affect the change of evapotranspiration of the watermelon in the autumn harvest． Photosynthetically active radiation，solar radiation，saturation difference，maximum temperature，average temperature，and minimum temperature are all significantly positively correlated with the daily evapotranspiration of watermelon． The correlation coefficients are 0．813，0．777，0．742，0．618，0．434，and 0．290，respectively． The relative humidity is significantly negatively correlated with the daily evapotranspiration of watermelon，with a correlation coefficient of －0．517． An empirical model for the daily evapotranspiration of watermelon seeds based on photosynthetically active radiation，solar radiation，and relative humidity is established． After an Ｒ2 of 0．706 is verified，the daily evapotranspiration of the autumn mini－watermelon in the greenhouse can be estimated．