China plans to release the last prisoner known to be held in connection with the 1989 Tiananmen Square protests after nearly three decades in jail, a US-based human rights group said Tuesday.

Miao Deshun, 51, is due to be released in October, the Duihua Foundation said, after serving more than 27 years in prison for his involvement in the mass demonstrations that were brutally put down by China's government.

Chinese courts originally gave him a suspended death sentence for arson after he and "four coworkers allegedly threw a basket onto a burning tank", it said.

Commuted to life imprisonment, his prison term was subsequently reduced several times, most recently this year, when authorities shortened it by 11 months for good behaviour.

"People who served sentences with him in the 1990s remember him as a very thin man who refused to admit wrongdoing and participate in prison labour", the statement said, noting that he had spent time in solitary confinement and been diagnosed with schizophrenia.

In 2013, Duihua said that China had released Jiang Yaqun, then 73 and thought to be the last prisoner convicted of counter-revolutionary offences in relation to the protests.

Chinese troops killed hundreds of unarmed civilians during the 1989 crackdown on pro-democracy protesters, by some estimates more than 1,000.