A WOMAN who chopped off her ex partner's penis to stop him impregnating other women will serve less than four years in jail.

Jian Chen was this morning sentenced to a minimum of three years and nine months in jail after pleading guilty to the manslaughter of her ex partner Xian Peng at her North Ryde home in February 2011.

The court accepted the guilty plea to the lesser charge, and withdrew the murder charge, because Chen was suffering from an "abnormality of the mind" due to mental illness and the belief Peng was going to take their child to China.

Chen believed her husband was "a conman" who wanted to impregnate wealthy woman to get money off them, the court heard.

The Supreme Court was told today Chen "laced" Peng's soup dinner with sleeping tablets and then castrated him when he woke several hours later.

He died in hospital the following night from his injuries.

Psychiatric reports tendered to the court say Chen now accepts the killing was wrong.

"But at the time he was a monster to me...his face was twisted," she told a psychiatrist in an interview done before today's sentencing.

Justice Monika Schmidt gave her a 25 per cent reduction for the guilty plea.

Allowing for the time she has already spent in jail, she will be eligible for parole in November 2014.

Chen dabbed at her eyes with a tissue throughout the one-hour sentencing and said "thank you Your Honour" when Justice Schmidt finished.