Evapotranspiration as a key link in the water migration of soil plant atmosphere continuum, it is of great significance for hydrological cycle process responses to global climate change. Accurate estimation of reference crop evapotranspiration (ET0) is necessary in water resource management and utilization in further. In numerous methods proposed for estimating ET0, among which Hargreaves equation only requires temperature data, but there is a large deviation between Hargreaves and Penman-Monteith equation in different regions, recommend by the Food and Agriculture Organization (FAO) of the United Nations as the standard equation for estimating ET0. It is an urgent need to revise Hargreaves coefficient so as to build the appropriate equation applying less climatic factor in the area. Meanwhile, it is possible to estimate potential evaporation in future climate change by using the revised Hargreaves equation which only needs future temperature data from RegCM4.0, which is the most reliable predictive data in further. The temperature based Hargreaves equation is confirmed to have a theoretical basis because temperature is the most important meteorological factor affecting ET0 through path analysis in the Minjiang river headwater region. The non-calibrated Hargreaves equation overestimated reference crop evapotranspiration, especially from April to October, and the maximum absolute deviation and relative deviation were 31.60mm and 29.7%, respectively. So it was necessary to adjust Hargreaves coefficient (AHC). AHC at the monthly scale, obtained by regression-based local calibration, minimized absolute deviation and relative deviation between the calibrated Hargreaves and the FAO Penman-Monteith equation, and the root mean square error (RMSE), modeling efficiency (EF), index of agreement (d) and coefficient of determination (CD), which were 3.76mm, 0.39, 0.8 and 0.84, were significantly lower than those of non-calibrated Hargreaves (14.66mm, 5.74, 0.51 and 7.81). The Hargreaves coefficient can meet the estimation requirements of ET0 in the study area, and it is available to predict ET0 in future. Under a future climate scenario, ET0 in the study area showed a increasing tendency, with a tendency rate of 5.6mm/(10a), the increase, however, would be slowed down.