A WA father who seriously assaulted his eight-month-old baby, leaving him with life-threatening injuries, has been jailed for more than four years.

A District Court sentencing hearing was today told the man, who cannot be identified for legal reasons, was left alone with his son for around three minutes in August 2014 when he became irritated that he could not settle him with a bottle.

The 41-year-old admitted to police he was "frustrated and tired" when he briefly "smacked the child to the head" but denied causing him grievous bodily harm.

However, after a three-day trial in January, a jury found him guilty of the charge.

The court was told the baby was weak, having trouble breathing and had blood in his mouth when his mother arrived home and the pair took him to hospital.

He was found to have collections of fluid on his brain and haemorrhages in both eyes and was flown to Princess Margaret Hospital for emergency treatment.

Medical experts found the only viable explanation for the baby's injuries was that he suffered a "significant force for a significant duration" or he had been shaken.

Defence layer Jim Sutherland said the assault was an isolated incident, and it was to the father's credit that he took immediate steps to get help for the child.

But sentencing the man to four years and four months behind bars, Judge David Parry said he had committed a "gross violation of parental authority".

"There can be no possible justification for shaking or hitting a child on the head, particularly a baby," he said.

"Children deserve love, care and affection from their parents, not physical abuse."

Judge Parry said the father had shown "absolutely no remorse" and appeared to see himself as the victim because his children were placed into State care as a result of his offending.

"(The child) not only experienced the trauma of the injuries but the trauma of being separated from his family," he said.

Judge Parry said the baby had made a full recovery from his physical injuries but was being monitored by doctors for future developmental or health issues.