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.
"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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