Taking a fire accident in at a pharmacy company in Chongqing on the September 2006 as an example, the causes of the electrostatic fire accident are analyzed by using the meteorological ground observation data and water vapor content inferred from GPS/MAT data, in combination with the production processes of the company. The results show that the extreme weather condition of high temperature and low moisture (air vapour content being 0.5 g/cm3, and surface temperature above 40 ℃) is the precondition for the electrostatic fire; another important cause is the incorrect measures for electrostatic prevention, which made a great deal of static electricity accumulated, produced spark discharge, and then led to the burning of a large amount of volatile petroleum aether vapour. In order to decrease the occurrences of the like electrostatic fire accidents, some precaution suggestions are given.