Creating thermally comfortable urban green space is beneficial to city vitality. When designing and building a green space, the layout of trees will influence its microclimate. This study aims to find suitable tree layouts that provide good outdoor thermal comfort for the commonly used 'open forest and grassland' type of green space. By using a validated numerical model, this study compared the microclimate and thermal comfort of a generic green space with 17 different layouts on a typical summer day in Shanghai, China. The comparison found that the air temperature and specific humidity difference within the site were limited to 1 °C and 0.8 g/kg, respectively, for all layouts. The shade provided by the trees greatly reduced the mean radiant temperature (Tmrt) and physiologically equivalent temperature (PET) by 20 °C and 11 °C, respectively. Trees provided a significant reduction in shortwave solar radiation, and evenly distributing the trees on the site only increased the fraction of shaded area from 11% to 14.7%. The major influence of tree layouts was on the wind distribution. A large low wind regime was created in the downstream area of trees and increase the PET value by up to 7 K. As a result, when considering the microclimate and thermal comfort of green space in summer, it is suggested to arrange the trees downstream of wind to avoid low winds at the site.