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.

The main parties failed to break the political deadlock after December's inconclusive elections.
The conservative Popular Party (PP) is favourite to win, but only narrowly.
In December the PSOE was runner-up to the PP. There is speculation that the PSOE and Podemos could form a ruling coalition after the 26 June vote.
The December election was a watershed for Spain, because the PP and the PSOE had previously alternated in power since the restoration of democracy in the 1970s.
Podemos (meaning "We Can") is now campaigning as Unidos Podemos, allied with the United Left party under communist leader Alberto Garzon.
Podemos leader Pablo Iglesias has offered to form a left-wing coalition government with the PSOE after the election. "We need to agree with them [the PSOE] so that we can have a progressive government," he said.
Breaking new ground, Podemos has presented its manifesto as an Ikea-style catalogue, with party members posing in different parts of an ideal house.
Podemos and the other leftists argue that the PP, under acting Prime Minister Mariano Rajoy, is discredited because of austerity and the chronic unemployment that has plagued Spain since the 2008 financial crisis.
The PP, however, points to Spain's improved economic performance as proof that its policies worked.
Judging by the current polls, Spain's result could mirror what happened in Greece, where the new left-wing Syriza party wooed voters away from the traditional Pasok socialists.
The fourth significant national party, according to the polls, is Ciudadanos (Citizens), a centre-right, pro-business party.
Ciudadanos hopes to occupy "a centre space in Spanish politics, with 3.5 million votes, which did not exist before in this country", its leader Albert Rivera said.
The party leaders will try to outdo each other in a high-profile TV debate on Monday, which could have an impact on millions of undecided voters.
Spain's Efe news agency reports that in this campaign the parties will hold fewer mass rallies - which are expensive - and focus instead on door-to-door leafleting and maximising their TV exposure.
They will also do less plastering of candidates' faces on public transport and in the streets, partly because of widespread voter discontent with politicians, Efe reports.
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.
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Campaigning has started 16 days ahead of a fresh general election in Spain, with left-wing Podemos edging ahead of the Socialists (PSOE) in opinion polls.