In terms of the daily fixed time surface meteorological data over recent 50 years, the standard layer daily sounding data at 07:00 BT and 19:00 BT from 2002 to 2006, the daily concentration data of main pollutants and sources from 2001 to 2006, the social and economic data from 1961 to 2006, the city development situations in 2002, and the Hohhot City planning maps from 1996 to 2010, the influence of urban development on atmospheric environment in Hohhot City was analyzed with the regional boundary layer numerical simulation method. The results have shown that: (1) The temperature in Hohhot City is higher after development than before, the temperature difference between urban center and surrounding areas increases and the heat island effect enhances. (2) The current convergence area increases and the capacity of convergence enhances in winter after development, as a result, the urban atmospheric pollution aggravates in winter. (3) The change of thermal and dynamic parameters of the underlying surface resulting from city expansion leads to air current fields to change remarkably, and then the capacity of convergence of pollutants strengthens. In short, although the green lands are increased after development, the higher and closer buildings and the change of underlying surface’s characters make the capacity of diffusion of atmospheric environments weaken and atmospheric pollutions become heavier.