Based on the data of 20 meteorological stations and two automatic stat ions and the data of 325m meteorological tower of IAP, characteristics of Beijin g urban heat islands (UHI) in July,2003 are analyzed. The results show that str ong urban heat islands occur in the night in July overBeijing under some specif ic weather conditions. The days with strong urban heat islands occupy about 1/3 of the total days. In addition, the characteristics and influencing factors of a strong urban heat island are analyzed in details. Results show that the UHI c enter lies over the urban area surrounding Tiananmen, Baijiazhuang, and the stro ng UHI occurs in the clear sky night with strong sunlight in the daytime. The weak wind and the temperature inversion are very important to the formation and maint enance of a strong UHI. But in the next daytime, the temperature inversion disap peared, and the amount of sunlight in northern suburb is more than that in the u rban area, which makes the surface temperature of suburban atmosphere rise faste r and higher than that of the urban surface atmosphere, and the intensity of UHI weaker. In addition, the strong urban heat island was simulated with MM5. And t he results show that when only the physical properties such as the roughness of the underlying surface were considered in MM5, simulated the urban surface tempe rature and the intensity of UHI don't accord with the observational data. Howeve r when the anthropogenic heat and urban heat storage are taken into account, the numeric simulation of UHI could be improved effectively. The anthropogenic heat and urban heat storage play very important role in the formation of the strong UHI in the night. 