Abstract: The synergistic effects of the extreme heat wave events in Beijing in June of 2023 and Paris in July of 2019 and urbanization on the urban thermal environment were simulated using the O (100 m) RMAPS-Urban model combined with data from automatic meteorological stations and ERA5 data.The mechanism was analyzed through the points of weather and climate background,topography and urban characteristics.The results show that the O (100 m) model combined with high-resolution land classification data can reasonably simulate the fine spatio-temporal distribution of near-surface meteorological elements and urban heat island.During the heat wave events,under the background of high-altitude ridge and clear air breeze,the contribution of urban factors to daytime air temperature rise increased by 0.20 ℃ and 0.55 ℃ in Beijing and Paris,and increased by 1.40 ℃ and 0.83 ℃ at night.During the heat wave events,the sensible heat flux in Paris is stronger than that in Beijing,resulting in greater contribution of urbanization to daytime temperature rise,while the higher ground heat storage in Beijing is the main reason for the greater contribution of urbanization to nighttime temperature rise.Urbanization has significantly slowed down the southerly mountain winds in Beijing at night,resulting in a "southeast higher and northwest lower" impact of urbanization on temperature in Beijing,while Paris has a smoother terrain and is not affected by valley winds.