Sean Zeisz, 20, admitted repeatedly assaulting Chloe Baker - who reached the "six chair challenge" stage in last year's series - on 3 October.
He also pleaded guilty at Liverpool Crown Court to assaulting her friend Toni Schumacher and a taxi driver.
Zeisz, from Liverpool, was jailed for a total of 16 months, of which he has already spent four months on remand.
Prosecutor Frank Dillon said that Ms Baker and Zeisz had been in a volatile relationship for about two years but it had become on-off.
In the early hours of 3 October, she and her friend Toni Schumacher bumped into Zeisz, who had taken both alcohol and cocaine that night, in a Liverpool bar.
Miss Baker spoke to him "to keep things civil" and later left the bar with her friend, who rang for a taxi.
When the former X Factor contestant tried to get into the cab, Zeisz grabbed her around the waist and then by the head and hair, the court heard.
He threatened to "knife" the friends and punched Miss Schumacher when she tried to help her friend. He also grabbed hold of the taxi driver's lip and gums.
A car, apparently summoned by Zeisz, turned up and although Miss Baker shouted that she did want to go, she "reluctantly" got into it.
Zeisz then repeatedly punched her in the face and bit her hand, Mr Dillon said.
Judge Brian Cummings, QC, said: "There are very sinister and serious aspects in this case."
Zeisz denied kidnapping her and that charge was dropped.
The former partner of an X Factor contestant who attacked her and two other people has been jailed.