A mother of two who cut off part of her former lover's penis and flushed it down the toilet has been sentenced to a minimum of three years and nine months in jail.

Jian Chen pleaded guilty to the manslaughter of Jin Xiang Peng, 48, after she drugged his soup with sleeping pills, bound his wrists and feet and stabbed him at her North Ryde home on February 9 last year. He died from multiple stab wounds.

The NSW Supreme Court accepted Chen's criminal responsibility for Mr Peng's death was reduced from murder to manslaughter because she was suffering from a "substantial impairment" of the mind at the time of the killing.

The court heard Chen thought Mr Peng, the father of her youngest son, was a "serial predator" and conman who got wealthy women such as herself pregnant in order to extort money.

Chen, who did not give evidence in court but gave an account of her relationship with Mr Peng to several forensic psychiatrists, said her former de facto partner emotionally manipulated her for many years.