Abstract: Clarifying the transpiration（ET） and change law of watermelon in soilless cultivation in greenhouse under different time scales and different irrigation conditions, thus to realize precise water management. Small watermelon was used as the test material, and the changes in ET levels and their relationship with environmental factors in watermelon under three irrigation treatments, namely 95%-100%（T1）, 75%-80%（T2）, and 55%-60%（T3）, were studied at different time scales. The results show that the peak value of ET on typical days from large to small is T1, T2 and T3. Under different temporal scales, the main factors affecting the ET of watermelon under different irrigation treatments are not exactly the same, and the degree of influence is also different, but solar radiation（R_S） is the main environmental factor affecting the ET of watermelon under different irrigation treatments. With the increase of the time scale, the main factors affecting ET of watermelon gradually decrease, while the significance of ET regression model of watermelon under different irrigation treatments gradually increases. When estimating the transpiration of greenhouse watermelon with environmental factors, direct measurement method should be used at the hourly scale. On a larger scale, it can be estimated by directly monitoring a few environmental factors in greenhouse.