A West Australian man who was sentenced to life in prison for the murder and sexual assault of his baby stepson has been found dead in his jail cell.

Mervyn Bell, 28, had been in the special handling unit at Casuarina Prison, 30 kilometres south of Perth.

He was found unresponsive in his cell on Tuesday afternoon and was later pronounced dead.

The death is not being treated as suspicious, and police will carry out an investigation for the WA coroner.

Bell was jailed after he took his 10-month-old stepson Charlie Mullaley from Broome in March 2013.

Over the next 15 hours, he sexually assaulted and tortured him in what the judge described as one of the most evil crimes he had ever encountered.

The child had injuries all over his body including burns, broken bones and severe lacerations.

At his sentencing hearing his lawyer, John Myers, said Bell was frequently confronted by prisoners in jail and believed he would be killed in prison.

Whilst awaiting trial in Albany Regional Prison, he was attacked with a hammer and suffered head injuries.

Earlier this year two other inmates, including the father of Carlton footballer Chris Yarran, were found not guilty of causing him grievous bodily harm over the attack.

Bell was also serving a five-year sentence for bashing his former partner, Tamica, on the night he took her son Charlie and murdered him.

He left her naked and bleeding by the side of the road in actions the District Court said were designed to humiliate her.

The sentence was being served at the same time as his life term.