It was unclear tonight why the man lit himself on fire, but he was airlifted to the burn center at the University of California, Davis Medical Center with burns to his hands, arms, abdomen and legs, sheriff's Sgt. Doug Pace said.
The man was undergoing surgery tonight but was expected to survive, he said.
The incident began when the sheriff's office received reports of a structure fire in the jail at about 7:35 p.m., Pace said.
Corrections officers discovered it was actually an inmate who was on fire and were able to quickly extinguish the blaze, Napa City Fire Battalion Chief John Callanan said.
The man, who was jailed Sept. 14 on suspicion of assault with a deadly weapon at Napa State Hospital, had wrapped himself in toilet paper and used a chord with a spark from a TV to ignite the toilet paper and light his clothes on fire, Callanan said.
The man was taken by ambulance to Queen of the Valley hospital in Napa before being flown to the UC Davis Medical Center, a county spokeswoman said.
The fire was contained to the man's cell, but smoke from the blaze forced the evacuation of about 100 inmates from the third floor to other parts of the jail, spokeswoman Elizabeth Emmett said.
Although the inmates were quickly relocated, Pace said the scene inside the jail was chaotic.
"There was a ton of people there," he said. "There was a ton of fire trucks. Napa State people were involved. Our people were involved. It's crazy. You just don't know what could happen."
He said things were "just now kind of mellowing out" at about 10:15 p.m. Detectives were still there talking to all of the inmates and witnesses, Pace said.
He said sheriff's deputies accompanied the man to the UC Davis Medical Center for security reasons but were not able to interview him tonight.
No other injuries were reported, and the structure did not sustain any damage.
Pace said it's not clear if the man had a cellmate, but most inmates with psychiatric issues are housed by themselves.
He did not know what crime the man was accused of committing.
"If they commit a crime in Napa State Hospital, they end up going to the jail," he said. "It's a hot topic."
The man has not been identified for privacy reasons, Emmett said.