Global climate warming and rapid urbanization have caused significant increases in the frequency, intensity, and duration of urban summer high temperature and heat waves. However, the knowledge of long-term characteristics, affecting degree, and driving mechanism of summer high temperatures on urban heat islands in highly populated megalopolises still need to be discovered. Beijing is the centre of the Beijing-Tianjin-Hebei metropolitan area and has experienced rapid urbanization in the past few decades. This study aims to elucidate the long-term trends of summer (May-August) high temperatures and explore the affecting mechanism of summer high temperatures on urban heat islands based on 43 years (1978-2020) daily meteorological observation data from the urban station (Beijing Station) and rural station (Miyun and Yanqing Stations). In the past four decades, the summer high-temperature days, intensity, and the yearly extreme maximal air temperature have shown significant increasing trends. Correspondingly, the initial time of summer high-temperature day was markedly advanced, and the end time of summer high-temperature day was significantly delayed. The urban heat island intensity of maximal air temperature significantly decreased, but the urban heat island intensities of mean and minimal air temperature slightly decreased during the summer high-temperature period from 1978 to 2020. The urban heat island intensities of maximal, mean and minimal air temperature during the summer high-temperature period were 0.73 ℃, 1.61 ℃, and 2.40 ℃, which were significantly higher than 0.09 ℃, 0.80 ℃, and 140 ℃ during the summer non-high temperature period from 1978 to 2020. The urban heat island intensity was significantly aggravated (more than 0.6 ℃) during the period with high-temperature compared to the period without high temperature, indicating that the urban heat island effect was amplified by summer high temperature. It is projected that summer high temperatures would become more frequent, and urban heat islands in Beijing would markedly increase under the background of future climate warming and rapid urbanization. These findings underline the serious heatrelated health risks facing urban residents.