Fire and rescue services from six stations are attempting to put out a blaze in a barn on Old Sodbury common.
Ground crews and two helicopters are currently attending.
Police have closed the road to the common.
Fire teams are pumping water to the site from St John’s Way.
A resident reported hearing a ‘loud explosion’ before ‘flames engulfed the sky’.
Avon Fire and Rescue Service and Avon and Somerset police have been contacted for further comments.