Kenneth Eriksson, a native of Kramfors in northern Sweden, was sentenced in 2006 to five years in prison for the killing.
But prosecutors have appealed the ruling and are now seeking the death penalty.
Prosecutors want Eriksson to be convicted for premeditated murder, according to the Allehanda newspaper.
Eriksson has been awaiting a new trial for two years and has written to Sweden’s Ministry of Foreign Affairs for help.
“The prison I’m sitting in is, according to others, probably the worst in Thailand. I’m not being subjected to physical violence, but mentally it’s tough to be here,” Eriksson wrote in a letter to Sweden.
He was convicted for having assaulted a woman to death in a hotel room in Pattaya.
Following the deadly assault, Eriksson then hid the woman’s body in a trunk which he then dragged out of the hotel and dumped in a field not far from the hotel.
He then checked out, but hotel security cameras had already captured images of him taking the victim out of the building.
Police in Thailand launched a manhunt for Eriksson, who eventually confessed and was arrested.