Based on the fatal forest fire data from 15 to 18 April 2003 in Diebu, meteorological data and EOS/MODIS data from May 2002 to April 2003,the Meteorological Conditions of the fire are analyzed.The result shows that higher air temperature and fewer precipitation in winter and spring,and long-term drought are the primary climate background of the fatal forest fire in the south of Gansu.The characteristics of 500hPa mean circulation of drought are as follow. There is a strong ridge at Xinjiang and a deep trough at the eastern coast of Eurasia. Cold front moved to south of Gansu and southerly wind developing is one of circulation features of fire in the south of Gansu. Continual increase of air temperature and continual decrease of humidity are the important conditions to fire happen and extend.It shows that the soil humidity can be reflected by EOS/MODIS,which is the most effective means to detect the happening of the fire, estimate the fire area of the fire.