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The reference evapotranspiration (ET0) has decreased over most of China under climate change over the past few decades, especially before the 1990s, which significantly affects the agricultural water requirements. In this study, the recent tendency of ET0 over China was determined and the issues of what causes this tendency and how this tendency would affect agricultural water demand were discussed. First, we evaluate ET0 (consisting of the aerodynamic term ET0_a and the radiative term ET(0_)r) using the Penman-Monteith method based on the grid-based China Meteorological Forcing Dataset (CMFD) from 1979 to 2015. Second, we analyze the trends of ET0 and five climatic variables using the Mann-Kendall test and the Sen's slope estimator. Finally, we assess the impacts of meteorological factors on ET0 by calculating dimensionless sensitivity coefficients and contribution rates. Based on the ET0 results and precipitation data, we also evaluate the aridity index (AI) over China to explore the impacts of the tendency of ET0. The result of the trend analysis of ET0 reveals that there is a significantly increasing trend after the 1990s over China, and this trend could be attributed to the increasing air temperature based on the results of the contribution rates, where ET0 is most sensitive to the specific humidity. The AI result reveals a decreasing trend over the study period, which indicates that the irrigation water supply could be appropriately reduced on the premise of proper and scientific utilization of water due to the more significantly increasing availability of water even though the water demand for evapotranspiration increased.