The blast at about 10 a.m. at Entertainment Fireworks was followed by a fire. Officials said firefighters put it out quickly.
Thurston County Sheriff's Lt. Gregg Elwin said emergency workers found three injured employees. One, described only as a 74-year-old man, died before he could be moved to a hospital.
The sheriff's office said the explosion did not appear to have been deliberate or criminal. The U.S. Bureau of Alcohol, Tobacco and Firearms will be involved in investigating the blast, officials said.
Entertainment Fireworks describes itself on its website as Washington's biggest pyrotechnics company. It says it has 21 magazines or explosives storage buildings at its facility.