Evapotranspiration (ET) is a link between soil-vegetation-atmosphere interactions, which is an essential process in understanding both water and energy balances of an ecosystem. Accurate quantitative analysis of its spatiotemporal changes with revealing sound underline forcing factors are of importance to promote the agricultural production and water management of a region. Based on the modified BEPS model, we used the remote sensing data and meteorological data to simulate ET in the Huai River Basin from 1981 to 2019 and analyzed its temporal and spatial changes. We further performed the attributional analysis to quantify its influencing factors with the help of the sensitivity index and contribution rate means. At last, we explored the main reasons for the lower ET in the special humid year of 2003 by numerical experimental methods. Results showed the following:(1) the long-term average annual ET of the Basin was 549.83 mm from 1981 to 2019, with the largest proportion in summer (47.63%). The evolution of the ET was an extremely significant increasing trend (4.41 mm/a, P<0.01) since 1981. Seasonally, the increase rates of ET in the other three seasons except winter exhibited obvious increasing trends (P<0.05). The increase rates of ET in the four seasons, in descending order, was summer, spring, autumn, and winter. Across the Basin, high ET values were observed in the central, eastern and southern regions. The center of gravity model showed that the area with high ET values moved significantly from north to south on the spatial scale. Attribution analysis showed that air temperature appeared the most sensitive driver for ET, followed by relative humidity and total solar radiation, leaf area index (LAI) and precipitation. However, the positive sensitivity of ET to the LAI gradually increased so that the significant increase in LAI contributed most to the interannual variabilities of ET (44.5%) in the Basin, followed by the increase of air temperature (25.93%). Correspondingly, we also found that LAI was the leading factor in ET of the spring, summer and autumn, while air temperature was the most significant variable for the winter ET. Finally, our numerical experiments indicated that the high relative humidity was the most important factor responsible for the Basin's low ET in 2003- a year with high humid, which is different from the attribution of the long-term ET changes. It is suggested to strengthen the attribution analysis of ET under extreme climate conditions in the future, so as to provide decision-making service for coping with global climate change more effectively. The results could provide scientific guide for understanding environmental influence to water cycle and the rational allocation of regional water resources in the Huai River Basin.