Urban heat environment issue has reduced the quality of life for residents, and urban green spaces can effectively improve the urban heat environment. However, at a fine scale, the cooling capacity of green spaces with different structures and the diurnal variation still require in-depth study. This paper, based on land use, vegetation cover, ground elevation, anthropogenic heat emission, and meteorological data, utilizes the UrbClim model to simulate high-resolution hourly temperatures in Beijing's urban area during summer high-temperature days. It analyzes the structural characteristics of urban green spaces at a fine scale and reveals the diurnal cooling effect differences among green spaces with various structures. The results indicate that the Urban Heat Island (UHI) within Beijing's urban area consists of a primary heat island and several smaller heat islands scattered around it. The northern part experiences higher temperatures, while the southeastern part is cooler, with the highest daytime temperatures ranging from 30.7 ℃ to 34.2 ℃ and the lowest nighttime temperatures ranging from 26.0 ℃ to 31.7 ℃. The structure of green spaces significantly influences the spatial pattern and fluctuation of air temperatures. Green spaces with high coverage exhibit a diminished cooling effect at night and can reduce the extent of daily temperature fluctuations, particularly when the greenery exceeds 80%, where the effect is most pronounced. The temperature stability of patchy and aggregated green spaces is stronger than that of linear and scattered green spaces. Vegetation types have different regulatory effects on diurnal temperatures; during the day, trees and shrubs have more significant cooling effect than grasslands, while at night, the warming effect of trees is more pronounced. Furthermore, at the hottest time of the day, green spaces with a high percentage of area, patchy and aggregated shape, and tree vegetation type have the best cooling effect, while at the coldest time of the night, these green spaces slow down the dissipation of heat accumulated during the day and have the most significant warming effect. The paper provides a scientific basis for relevant departments to carry out urban planning work and improve urban heat environments for decision-making and management.