Stephen Gibbs, 45, initially denied the attempted murder of Emma Brown, 49. He was arrested after a 999 call to their £700,000 home in Barry, South Wales, on January 30.
Ms Brown suffered knife wounds to her face, arms and stomach and required surgery.
Gibbs was due to go on trial at Cardiff Crown Court but changed his plea at the last minute on Monday.
He confirmed his name and details before admitting attempting to murder Ms Brown.
Judge Richard Twomlow said he would sentence Gibbs after a psychiatric report had been carried out.
He warned the defendant he faces a “lengthy” prison sentence for the attack.
He said: “I need to consider the question of dangerousness.
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The couple had been together for 11 years.
Ms Brown was a deputy manager working for aircraft handling firm before winning £5.5m on the lottery in 2017.
The couple previously owned a home in another part of Barry which they rented out after Ms Brown’s lottery win.