Mohammed Ali Abboud is on trial for murdering Agnieszka Szefler at the home they used to share in Bridge of Earn, Perthshire, in January.
The jury were shown video footage of Mr Abboud telling detectives Ms Szefler "fell" with the knife in her hand.
The 57-year-old denies all the charges against him.
The High Court in Edinburgh heard Mr Abboud told officers that Ms Szefler's temper changed just before she had her period and that she often threw household items at him during this time.
He said the 27-year-old teacher sustained serious injuries during the struggle on 23 January.
He said: "She fell. She had the knife in her hand."
The video footage of the police interview was shown on the fourth day of proceedings against Mr Abboud, who also denies a charge of attempting to defeat the ends of justice by covering up his role in his former partner's death.
In the footage, a detective told Mr Abboud that he did not believe his claims that he did not murder Ms Szefler.
The policeman then asked the accused to admit that he murdered her.
But Mr Abboud said the pair got into a struggle and he was trying to stop her from causing any of them serious injury.
He said: "I was trying to stop her. It got out of hand."
Mr Abboud's lawyers have lodged a special defence of self defence.
The trial, before judge Lord Uist, continues.
A man accused of stabbing his ex-partner to death told police she came at him with a knife and they started to struggle, a court has heard.