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Reference evapotranspiration (ET0) is often used to make management decisions for crop irrigation scheduling and production. In this study, the spatial and temporal trends of ET0 in China’s most northern province as well as the country’s largest agricultural region were analyzed for the period from 1964 to 2013. ET0 was calculated with the Penman- Monteith of Food and Agriculture Organization of the United Nations irrigation and drainage paper NO. 56 (FAO-56) using climatic data collected from 27 stations. Inverse distance weighting (IDW) was used for the spatial interpolation of the estimated ET0. A Modified Mann-Kendall test (MMK) was applied to test the spatiotemporal trends of ET0, while Pearson’s correlation coefficient and cross-wavelet analysis were employed to assess the factors affecting the spatiotemporal variability at different elevations. The results from this study showed a clear decreasing trend for annual ET0 from the low elevation plain area to the high elevation mountainous area. Over the past five decades, ET0 in Heilongjiang Province decreased in all seasons, except for the winter months, during which a steady increase in temperature was found. Elevation played an important role in estimating ET0 in this higher-latitude region, while relative humidity was the most relevant meteorological factor that affected the spatiotemporal variation of ET0 in the province. Overall, the findings from the study suggest that winter ET0 in a high altitude region will continue to increase in the future as climate change persists, which could worsen spring droughts and irrigation management for semi-arid areas in the province.