The characteristics, including the distribution in a month and the duration of Beijing estival muggy weather, is gained here by analyzing the daily maximum and minimum temperature in summer season(June, July and August) from 1951 to 2000. It revealed that the occurrence probabilities of the hot and muggy weather during the summer in Beijing are, respectively,1/10 and 1/20, which indicates Beijing is a suitable city for the Summer Olympic Games. The hot weather mainly occurs in late June and the whole July, with two maximum frequencies in the period of 26 to 30 June and 21 to 25 July, respectively. The muggy weather mainly occurs in the middle of July and early August, with two maximum frequencies in the period of 26-31 July and 6-10 August, respectively. There are many hot weather days but few muggy weather days in June . And there are very few hot weather days but several muggy days in August. The hot weather lasting more than 2 days occurs at least once a year. The occurrence frequencies of the hot and muggy weather lasting more than 3 days are 0.84 times/ year and 0.88 times/year respectively. The average duration of the hot and muggy weather are both 3 days. In the middle and late August, the hot and muggy weather seldom occur. That time is a good period for holding the Olympic Games.