Sustainability of irrigated agriculture depends primarily on the efficient management of irrigation water. Understanding crop evapotranspiration (ET_c) is essential in planning the most effective use of water resources in the arid northwest China. ET_c is not only the important component of water balance, but also an important requirement for proper constitution of irrigation strategy. In this study, the middle reach of Heihe River was selected as the study area which has the largest agricultural oases in China. Agricultural development has over 2000 years history own to its flat land. Irrigated spring wheat is the main crop in this region. With the increase in population growth and farmland area is 2452.1 km² agricultural water requirement in the middle Heihe River basin is rapidly increasing. This results in the conflict between the agricultural water demand and ecological water demand. Ecological environment deteriorates due to vegetation disappearing at a rapid rate in large areas in the region. In the field, when the crop is small, water is predominately lost by soil evaporation, but once the crop is well developed and the canopy completely covers the soil, transpiration becomes the main process of water loss.. The objective of this study was to map the spatial distribution of water requirements and analyze the relationship between the spatial variation of water requirement and main meteorological parameter during 1970-2009 in the Heihe River basin. First, reference crop evapotranspiration (ET₀) was calculated using Penman-Monteith equation with daily data obtained from 14 meteorological stations in and around the study area. Secondly, the crop coefficients (K_c) of spring wheat in different growing stages were adjusted according to the data obtained from the field studies. Finally, we estimated the spatial distribution and variation trend of the water requirement of spring wheat in the middle reaches of Heihe river basin with the use of Geographical information system (GIS). The results showed that the trend of crop evapotranspiration (ET_c) gradually increased from south to north,the ET_c varies from 573 mm to 781 mm during the entire growing season. The ET_c in Gaotai, Zhangye, Linze, Minle, Shandan, and Jiuquan is 731.26, 686.88, 598.24, 728.89, 719.77 and 713.59 mm, respectively. The ET_c of mid-season stage is the largest percentage during the entire growing season, accounting for 51.67%, 51.11%, 50.96%, 51.24%, 50.83% and 50.77% of total ET_c at six meteorological stations, respectively. Temperature, precipitation, wind speed, min relative humidity and sunshine duration were the dominant meteorological parameters influencing ET_c, and the relative influences of temperature, sunshine duration, wind speed, precipitation and min relative humidity are 57.29%, 26.92%, 15.15%, 1.41% and 0.78% respectively.