Local climate changes are exacerbated in urban areas due to the rapid urbanization, which will accordingly worsen the urban thermal environment and cause series of eco-environment problems. Urban green spaces are multifunctional and can reduce temperature and mitigate the urban heat island effect. Previous studies have found that urban microclimate, vegetation types and the characteristics of green patches are all contributed to the green spaces cooling effects. Quantifying the relationships between these factors and green space cooling effects will help to guide the urban green space planning and design, and accordingly help to mitigate the urban thermal effect. In this paper, five woodlands were selected as the research spots, which have similar vegetation structure, and another spot which was mainly covered by cement was selected as the reference point. All of the six observation sites are in the Purple Mountain along the direction of Lingyuan Road to Dr. Sun Yat-sen's Mausoleum. The HOBO meteorological stations at fixed observational spots were used to get meteorological factors such as air temperature, relative humidity and wind speed. The LAI-2200 and fisheye photograph were used to capture the Leaf area index and sky view factor respectively, and the handheld GPS navigation was used to get the elevation of each study spots. The relationships between temperature reduction and related factors were analyzed using correlation analysis. The results indicated that: (1) The air temperature of each spot showed the same changing trend; (2) Compared with the concrete (reference point), all of the five woodlands had obvious cooling effect during daytime with maximum temperature reduction in the period of 13:00-16:00. However, it seems the cooling effect is weak at night, which may be because that woodlands have heat preservation effect during night; (3) The temperature reduction of all the research points showed the increasing trend with the rising of air temperature,and decreasing with inducing of relative humidity or wind speed; (4) The impact of leaf area index, sky view factor and elevation was different at different period of investigation time. Leaf area index and sky view factor had a comparatively obvious effect on the temperature reduction during 9:00-19:00, but the effect became weaker after noon (13:00-15:00) due to highest temperature of a day. However, during the night, elevation became the key impact factor and it showed a significant positive correlation with temperature reduction. Through the field investigation of the study area, the cooling effect characteristics were captured and the relationships between cooling effect and main related impact factors were quantified. The results can help us understand the factors contributing to the green space cooling effect and provide practical guidelines for planning urban green spaces to mitigate the urban heat island.