Atmospheric mixing depth is one of most important parameters in air quality potential forecasting. Based on Holzworth’s fundamental and Wang’s stepwise approach, by using daily meteorological data from Beijing Observatory during 1970-2007, the maximum mixing depths (MMD) in Beijing Area were calculated and their variation characteristics with time were analyzed. The results showed that, from 1970 to 1998 the annual mean MMD had a decrease trend and got a minimum value in 1998, then from 1998 to 2007, the annual MMD turned into an increase trend. The seasonal variation showed the mean MMD being low in winter and high in summer. And the air quality and MMD frequency distribution revealed there is a remarkable negative correlation between them.