Evapotranspiration (ET) links water and energy cycles, and is one of the most important water fluxes in the rain-fed agroecosystem of the Loess Plateau, China. Accurate measurements of ET in this semi-arid agroecosystem are critical to gaining a better understanding the interactions between terrestrial ecosystems and atmosphere, and tackling climate change scientifically. This study was conducted at the Dingxi Arid Meteorology and Ecological Environment Experimental Station (DAMES) in 2010. The eddy covariance system and microclimate gradient system were used to observe ET and environmental conditions, respectively. We analyzed the diurnal and seasonal dynamics of ET and environmental factors influencing ET throughout the spring wheat agroecosystem in the semi-arid Loess Plateau. The results showed that the diurnal variation of ET in this agroecosystem can be graphed as a line with a single-peak which occurs around noon with lower ET rates observed in the morning and evening; also, seasonal ET peaked in August (0.22 mm/h). Daily peak ET values in the growing season were larger than those in non-growing season. During the observation period, the maximum daily ET rate of spring wheat farmland was relatively low, which may be related to the dry climatic conditions with low levels of precipitation typical of the semi-arid area of the Loess Plateau. ET varied significantly from season to season in the spring wheat agroecosystem and ET dynamics were closely related to the seasonal distribution of precipitation. Monthly ET peaked during July and August because relatively more rain was received during these two months. Annual ET (318.0 mm) in 2010 was slightly lower than annual precipitation (332.3 mm); the ratio of ET to precipitation was 95.7%. ET during the non-growing season was significantly lower than that during the growing season (April to September), with a ratio of non-growing season to growing season ET of 0.26. Total ET in the spring wheat agroecosystem increased exponentially with increases in either soil water content and/or air temperature (lower than 26 ℃). ET responded to changes in relative humidity, solar radiation, and wind speed in a quadratic curve manner with a pattern of first increasing and then decreasing. Net radiation was the major environmental factor controlling ET in the semi-arid agroecosystem of the Loess Plateau, followed by soil water content.