Abstract: Abstract: Urban heat island refers to a phenomenon that the temperature of the central unban is much higher than that of the surrounding suburbs. It is a harmful environmental problem which is mainly caused by urban development and the increase of population. The problem of thermal environment in the large and medium-sized cities is increasing day by day, which has already affected the daily life and the health of residents. As an essential part of ecological system, urban green patch plays an important role in adjusting urban thermal environment, improving urban climate and maintaining urban ecological environment by changing atmospheric water thermal cycle. In our developing country, rapid development of urban economy leads to a large number of land occupied by urban construction. The development of urban should not only be considered in the economic construction, but attention should be paid to protection of the ecological environment. At present, large quantities of researches have proved that expanding the scale of green space is the most economic and effective measure to alleviate urban heat island and improve urban thermal environment. However, less consideration has been taken to location differences of green space cooling effect. Reasonably projecting the construction of urban green space can balance the relationship of economic development and ecological environment and relieve the heat island effect effectively, according to the characteristics of different locations. The influence of locational factors on ecological environment can not only optimize the distribution of green space resources, but also resolve conflicts between the construction of urbanization and the protection of ecological system. At present, the research about the relation between urban green space and the heat island effect has been very mature, but the study of cooling effect of different regional green space is less. This study took Haidian district in Beijing as the study area, with 100m grids as evaluation unit. To select sorts of evaluation indexes from the perspective of green space demand, establish green demand degree of evaluation indexes at the consideration of green space present situation, the thermal environment status and the contribution to the heat produced by human activity factors. Construction of green space demand composite index and division ecological niches on this basis. Then we divided the study area into 3 types of ecological niches, including high green space demand area, mid green space demand area, low green space demand area. We obtained the best green space area in different ecological niches by conducting researches of the green space cooling effect on the surrounding environment in different ecological niches. The best green space area is 1.4-22 hm2 in high green space demand area, 1.6-24 hm2 in mid green space demand area and 0.24-17 hm2 in low green space demand area. The result can provide a certain reference for construction optimization of green space area, and an effective method to divide ecological niches which is more reasonable than that from the angle of geographic space.