The study on the spatio-temporal variation of reference crop evapotranspiration and its causes in Huang-Huai-Hai region is an effective way for improving the utilization efficiency of regional agricultural water resources. Based on the daily observation data at 364 meteorological stations in Huang-Huai-Hai region during 1961-2018, the reference crop evapotranspiration (ET0) was calculated by using FAO Penman-Monteith equation. And on this basis the spatio-temporal variation of ET0 and contribution of influencing factors were analyzed by using trend analysis, sensitivity analysis and contribution rate analysis. The results are as follows: (1) The annual mean ET0 was 978.7 mm in Huang-Huai-Hai region during 1961-2018, and it decreased gradually from the central to surrounding areas. The annual mean ET0 decreased significantly with the rate of 12.5 mm·(10 a)-1 from 1961 to 2018, and the decadal variation of ET0 was obvious, especially before the 1990s. Meanwhile, the annual mean temperature increased significantly, and the wind speed, sunshine hours and relative humidity decreased significantly, but their climatic tendency rates in each province and area of Huang-Huai-Hai region were different slightly. (2) The sensitivity coefficients of ET0 to mean temperature, wind speed and sunshine hours were positive, while that of ET0 to relative humidity was negative. (3) The contribution rates of mean temperature and relative humidity to ET0 were negative, while that of wind speed and sunshine hours were positive, and the absolute value of contribution rate decreased in turn from wind speed, sunshine hours, mean temperature and relative humidity. (4) The decrease of wind speed and sunshine hours was a main reason for ET0 decrease in Huang-Huai-Hai region, but the reason of ET0 decrease was slightly different in each province and area.