A woman who was in a psychotic state when she killed her 18-year-old son, stabbing him as they argued at a Perth bus station, has been jailed for six-and-a-half years.

Tanya Mynette Ugle, 37, stood trial charged with murder before Supreme Court of WA Justice Stephen Hall on Monday but prosecutors accepted her guilty plea to manslaughter on Tuesday.

The court heard she pierced Jamahl Jordan Ugle once in the chest with a 15cm blade she had hidden in her pants after being kicked off a bus for disorderly behaviour in Fremantle on November 23, 2016.

Ugle was swiftly arrested after transit guards chased her to a nearby shopping centre and handcuffed her before police arrived.

One of the officers told the court she was swearing and screaming incoherently as they tried to inform her of her rights, was "clearly drug affected" and claimed her son "pushed me ... beats me".

The court heard she had a long history of mental health problems after a difficult childhood, which included abuse and being placed in foster care.

Jamahl's father died in unknown circumstances and a son Ugle had with another partner is also deceased.

She had for many years been living on the streets where she was attacked physically and sexually many times, defence counsel Anthony Eyers said.

She carried weapons to protect herself and, ironically, Jamahl.

Through Mr Eyers, Ugle told Justice Hall: "I'm sorry for what I've done".

"It's the last thing I meant to do.

"I wish I could be a better mother and change the past."

Justice Hall said Ugle had been caught in a vicious cycle whereby she abused drugs, including injecting methamphetamine, in an attempt to cope with her mental illness but it made it worse.

She had been diagnosed with paranoid schizophrenia but was untreated in the lead-up to the killing.

Justice Hall said her mental illness was one but not the only cause of the attack and it reduced her culpability to some extent.

However, she had been able to rationalise and bid for a better outcome for herself when police told her - as Jamahl was clinging to life in hospital - that she would be charged with attempted murder, arguing she should instead be charged with unlawful wounding.

"I didn't stab him that bad," she told the officers.

"He wasn't bleeding much after I cut him."

Ugle, who has two other children, will be eligible for parole in May 2021 after she has served four years and six months, considering she has been in jail since the crime.

When Justice Hall handed down the sentence on Friday, family members sobbed and shouted in anger.

"Oh bull****. You made yourself the victim," one said.