Potential evapotranspiration is an important parameter to measure the evolution of hydrothermal resources in a changing environment. It is essential to explore the history of the potential evapotranspiration rules and constraints to predict the trend of agricultural water and make relevant management decisions. The daily meteorological data from 23 meteorological stations in Liaoning province from 1966 to 2015 were analyzed in this study using the Penman-Monteith model recommended by FAO and the partial correlation test. Spatial and temporal characteristics of potential evapotranspiration and its influencing factors in Liaoning Province in the last 50 years were identified. The results show that the annual average potential evapotranspiration of all monitoring stations in Liaoning Province is 453-1043 mm, with the multiyear trend decreasing at a rate of 1.43 millimeters per year and suddenly increased in 2003. In addition, the temporal and spatial differences of potential evapotranspiration in northwestern Liaoning are significant. It is shown that the potential evapotranspiration gradully decreases from northwest to southwest Liaoning on the interdecadal scale. Additionally, the highest potential evapotranspiration occurs in the summer, followed by spring and autumn, with the lowest occurring in winter, which is on the seasonal scale. The daily maximum temperature, the daily minimum temperature, and the decrease of sunshine hours resulted in the decrease in the potential evapotranspiration in Liaoning in the last 50 years. Together, the research results can provide some theoretical support for the optimal allocation of water resources and evaluation of the degree of regional wetness and dryness.