One man died instantly after the explosion at 6pm at UniThai Shipyard and Engineering, while another worker was seriously injured.
An initial police report said the blast could have occurred as a result of an acetylene gas leak at a dock in which a ship was undergoing repair work.
Witnesses said the dead worker, Sanit Phromebuppha, was hit by a flying iron bar and fell off a 10-metre building while others said the blast blew the workers five metres into the air.
A survivor, Phaithoon Namprapha, said he and around 20 workers were cutting pieces off a large metal sheet near the dock and the blast "came out of nowhere". He said sparks from the blowtorches could have been ignited by escaping gas.
Laem Chabang police gave similar accounts to the survivors but declined to provide details into the cause of the explosion until a complete crime-scene investigation is completed next week.