Rapid urbanization results in the increase of population density of large cities. And urban heat island effect becomes severe. Park green spaces, as an important component of the city ecosystem, play a significant part in alleviating heat island. We choosed 8 large parks of Tianjin center city as empirical objects, used single window algorithm and analyzed their cooling effect based on Landsat8 ETM+image in 2013. The results show that the coverage areas of the parks and vegetation coverage ratio are the most important factors influencing the urban heat island effect, and also water ratio has a great influence on the surface temperature. The maximum cooling intensity and cooling distance of the 8 parks differ largely with each other. The cooling intensity temperature difference is about 2.72℃ and the cooling distance is 195 m. Parks with larger area has more obvious cooling effect, especially when the water ratio is higher than 30% or the vegetation coverage higher than level 10. A model of Parks' cooling effect is deducted based on the conclusion of the study. The results of the research provides effective technical support for the planning of park green space in Tianjin center city and similar high density urban areas.