Urban heat island (UHI) has an important effect on urban eco-environment, living and production, and physical and mental health of the residents. In addition, urban warming especially summer heat wave caused by UHI significantly affects many aspects of the global economy, such as energy and water consumption, transportation, and social economy. Understanding of long-term trend of urban heat island intensity and its climatological driving mechanism will help the rational urban planning, urban livable construction, and urban sustainable development. Beijing is the center of the Beijing-Tianjin-Hebei metropolitan area, and has experienced a rapid urbanization process in the past few decades. This study aims to elucidate the long-term trends of UHI intensities of mean air temperature, minimum air temperature, and maximum air temperature and the climatological driving mechanism based on 50 years (1967-2016) meteorological observation data from urban station (Beijing station) and rural station (Miyun station). In the past five decades, the UHI intensities of mean air temperature, and minimum air temperature showed a significant increasing trend with the increasing rates of 0.29℃/decade (r~2=0.59, P<0.001) and 0.45℃/decade (r~2=0.62, P<0.001) respectively. In contrast, no marked variability trend was observed for the UHI intensities of maximum air temperature. Statistical analysis has shown that relative humidity, wind speed, and sunshine duration decreased significantly and air temperature increased significantly in Beijing over the past 50 years, which is conducive to the formation of UHI and the enhancement of UHI intensity. Multiple stepwise linear regressions showed that relative humidity, maximum wind speed, and atmospheric pressure were the key climatological factors controlling UHI intensities of mean air temperature and minimum air temperature, which could explain 92.4% and 87.6% of variabilities respectively. Atmospheric pressure, relative humidity, and sunshine duration were the key climatological factors controlling UHI intensities of maximum air temperature. Under the background of global warming and rapid urbanization, UHI effect in Beijing will further intensify, resulting in more frequent and prolonged summer urban heat waves, which will seriously endanger urban residents' production, life and health. Therefore, it is necessary to consider the effects of UHI on the future urban planning and construction. By optimizing urban layout, carrying out reasonable road system planning, energy planning and ecosystem planning and other measures, we can alleviate UHI effects and reduce high temperature and heat waves harm caused by UHI.