Abstract: Urban forest is an important green infrastructure alleviating urban heat island. This study was aimed to explore the internal and external factors influcening the cooling impacts of urban forest and to understand the mechanisms of such influence. It provides an scientific basis for optimization of urban forest structure and alleviation of the effects of urban heat island.The internal factors that affect urban forest cooling effect at individual tree scale were tree species, canopy structure and crown shape, while at stand scale, the factors mainly contained forest type, forest structure, green biomass and shape and size of the stand, moreover, the cooling effect of urban forest was studied mainly by remote sensing method at urban scale, and the representative indices included NDVI, LAI and landscape ecological indicators. In addition, the external factors that affected the cooling effect of urban forest included non-biological factors such as climate and ground types, as well as forest management. The internal and external factors affected the cooling effect of urban forest jointly by affecting the flow and exchange of energy inside and outside the forest.To alleviate the effect of urban heat island, on the one hand, it is necessary to strengthen theoretical research, attach importance to the theory of temperature field, and study the relationship between stand structure and energy transmission in urban forest by combining ground observation and remote sensing. On the other hand, it is important to strengthen the management of urban forests, optimize the stand structure from the perspective of promoting heat exchange, and rationally match buildings, forests and water to maximize the cooling effect.

Key words: heat island effect, urban forests, cooling effect, canopy configuration, transpiration effect, human disturbance, leaf area index