Firefighters put out the blaze, at Apai Bori Rak in Muang district, before it spread to a 7-Eleven shop next door.
The shop, that makes vinyl signs and stickers, is owned by Nanthapong Boonyome, 30, who was injured along with his five workers. Two of the six were severely injured.
The injured were taken to Piyarak Hospital and Phatthalung Hospital.
Some of the workers told police that Nanthapong was drinking with his employees at the back of the shop and one of them was grilling fish when the explosion occurred at about 11.40pm.
Police believe the charcoal was put too close to the cans of paint thinner and the heat caused the cans to explode.