Based on the data of 20 meteorological stations from 1990 to 2004 in Beijing, the characteristics and the variations of the urban heat island effect over Beijing are analyzed, and the relationship among the air temperature, the urban heat island effect, the ground temperature and the urban-rural ground temperature difference are also analyzed. The results show that strong UHI occurred in the night, but in the daytime UHI is not obvious. As a whole, UHI intensify year by year in the night, but in the daytime it is not obvious. In the night, the higher the surface temperature is, the stronger UHI becomes, and the greater the difference in urban-rural ground temperature becomes, the higher the ground temperature is, But in the daytime, all these relations are not so obvious as those in the night. The results can be used as scientific references to the development planning of Beijing city and the protection from high air temperature disasters of Beijing city.