Abstract: Urbanization has triggered a series of environmental problems, such as the urban heat island (UHI). Previous studies have shown that the cooling and humidifying effects of urban forests can alleviate the UHI effectively and provide comfort and healthy environment for outdoor activities of the public. In this study, we chose 17 plots of urban forests in Beijing’s urban parks as our research targets. The forest area and the distance from inside to the edge of the forest (L) were used as parameters to represent forest scale, and leaf area index (LAI) was used to characterize forest structure. The relationships between forest area, L, LAI and air temperature reduction percentage (dT),relative air humidity increase percentage (dRH), thermal humidity index reduction percentage (dTHI) were explored, respectively. The results showed that: 1) with forest area increasing, the cooling and humidifying effects were strengthened, which led to 61.67% of dT, 69.44% of dRH and 55.63% of dTHI. 2) When LAI of urban forest increased, the cooling effect was strengthened, contributing 58.98% of the cooling effect. 3) There was an interactive impact between forest area and LAI on dT, i.e., as the forest area increased, the effect of LAI on dT was weakened. 4) dT ascended with the increase of L, and 57.26% of the dT was caused by the latter. Therefore, with the purpose of relieving the UHI, it is important to develop large areas of urban forests and maintain high forest coverage during the urban planning in order to enlarge their cooling and humidifying effects.