Contestant Colin Cloud convinced the 42-year-old British singer to jab Cowell with the fake weapon after an audience member chose the white-handled rubber knife from four other knives with real blades.
"It's not working!" Mel B announced after stabbing Cowell.
Cowell said while giving Cloud feedback that Mel B didn't realize the knife was fake before attempting to plunge him in the chest.
"I'm just sort of collecting myself a little bit at the moment," the 57-year-old television personality said. "I nearly got murdered on live TV."
Tuesday's act followed Mel B and Cowell's spat on the show's August 22 episode. Mel B threw a cup of water on Cowell and stormed offstage after he made a joke about her wedding night.
"I kind of imagine this would be like Mel B's wedding night," Cowell said after a contestant had a technical malfunction. "A lot of anticipation, not much promise or delivery."
Mel B later told E! News Cowell's comment was "so rude and inappropriate." She explained she was feeling emotional from contestant Evie Clair's performance when Cowell made the quip.
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"My journey tonight was all over the place. But I'm a very real, honest person, so what you see is what you get," the singer said.
Mel B filed for divorce from Stephen Belafonte in March after splitting from the film producer in December after nearly 10 years of marriage.
She later obtained a temporary restraining order against Belafonte after alleging he was abusive.
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