A New South Wales man has been jailed for up to eight years for bashing his partner's four-year-old son to death.

Nathan Forrest pleaded guilty to the manslaughter of Bailey Constable, who died in April 2011 near Warren, in western New South Wales.

Justice Elizabeth Fullerton has sentenced Forrest to eight years in jail, with a non-parole period of six.

The judge said Forrest was in a position of authority over the boy, who was sick and recovering from other injuries the night he was bashed.

The child's mother, Jessica Constable, previously told the Supreme Court Forrest had injected himself with speed and hit the child in the bathroom after the boy wet the bed.

She said after her son lost consciousness, Forrest shook him then tried to resuscitate him.

She also said Forrest wanted her to tell people they found Bailey face-down in the bath.

Forrest will be eligible for parole in 2017.