A Taiwanese woman who set off the island's deadliest fire in decades to get back at her boyfriend escaped the death penalty Friday as the court handed down a life sentence.

Last October's inferno in the southern city of Kaohsiung raged through multiple floors of a dilapidated 13-storey apartment block for hours, killing 46 people and injuring 41.

Authorities had said the blaze started when the defendant Huang Ke-ke, 51, left unextinguished incense ashes on a sofa before leaving the building.

Huang was indicted on murder and arson charges in January, with prosecutors seeking her execution.

Prosecutors said she deliberately started the fire to "embarrass" her boyfriend, who she suspected of cheating, and had shown no remorse for her actions.

Kaohsiung district court on Friday convicted Huang on charges of arson and negligence leading to death, and sentenced her to life in prison.

"The defendant didn't have motive to inflict damage on other residents in the building... the murder offence didn't stand because she didn't deliberately" cause the deaths, the court said in a statement.

Huang admitted lighting sandalwood incense to repel mosquitoes. But she gave inconsistent statements on what she did before leaving her room, according to prosecutors.

She initially claimed she threw the incense into a bin, but later said she could not remember what she did.

The blaze highlighted concerns over lax safety standards in Taiwan and exposed the poor living conditions of the elderly in a rapidly ageing society.

Prosecutors said they would appeal the ruling.