Daniel Bragg, 37, hit Julie Cook, 44, so hard he broke her left eye socket in the assault, police said.
Ms Cook fell and hit her head on the floor, causing a serious injury. She died in hospital two days later.
Bragg, who admitted manslaughter, was sentenced to five years and four months in prison.
He initially denied the killing, but changed his plea on the second day of his trial at Winchester Crown Court.
Bragg was overheard by friends four days before Ms Cook was killed saying "one of these days I'm going to kill her", Dorset Police said.
A Domestic Violence Protection Order had been made against Mr Bragg in April stopping him from contacting or going to Ms Cook's home, the force said.
However, the couple, from Bournemouth, remained in contact and on 28 April 2016 - just four days before the assault - a friend of the couple heard Mr Bragg say something to the effect of "if we carry on like this I will end up killing her".
The couple had been at Pokesdown Railway Station when they started arguing and Ms Cook walked away.
After punching her, Bragg initially walked away but returned to help Ms Cook, who was unconscious on the concrete platform, and took her back to her flat.
The couple had been together for about four years before the incident and their relationship was described as "volatile".
Det Insp Neil Phillips said: "There was no justification for such violence. Bragg and Julie clearly had a volatile relationship and efforts had been made to prevent further abuse.
"However, Julie and Bragg continued to maintain contact and she tragically died as a result of his violent behaviour."

The Hydrus Group, an oil and gas service company, has started work on a specialist manufacturing base at the Brechin Business Park.
It is forecast the new development will bring tens of millions of pounds into the local economy.
The new facility should be complete by spring 2015 and bosses say it will create 100 jobs over five years.
Hydrus presently employs 50 people in Brechin at a site on Eastmill Road, which will trade in tandem with the new premises.
The facility will include capacity for specialist engineering structures to be built on site, ready for export.
Mark Williams of Hydrus said it was a "natural step" for the firm to expand in the area.
He said: "The growth of the energy sector in Aberdeen is spilling over into Angus for the area's benefit.
"Brechin is a great location for us. We are creating huge structures for the oil and gas sector.
"Good transportation links are essential, so the proximity of the deep sea access at Montrose Port is a significant benefit, particularly as we look to expand our export markets."
Angus provost Helen Oswald said the new facility was "great news for Hydrus and the county".
She said: "Angus Council is delighted to be working with such an innovative company, whose enterprise and ambition is bringing jobs and economic benefit to Angus."

The 36-year-old Czech national was arrested on Saturday after the body of Rebecca Johnson, 26, was found in the Finnish village of Kuttanen.
Ms Johnson was a member of the Santa Safari team which works with tour operator Transun Travel to organise Christmas-themed excursions to Lapland.
The couple lived in Scotland before moving to the Arctic for seasonal work.
Ms Johnson was from Burntisland in Fife.
A Finnish police statement said: "Lapland District Court has today imprisoned the suspect of the homicide, according to the claim of the Lapland Police Department.
"A 36-year-old Czech citizen is suspected of killing his 26-year-old long-term partner. The victim is a female citizen of Scotland. The crime is investigated as a murder."
It is understood members of Ms Johnson's family have since travelled to Lapland.
The police statement said: "The suspect has been co-operative but hasn't been able to clarify a specific motive for his suspected act.
"Police will continue the investigations by hearing the witnesses and the suspect, and by conducting technical investigations."
The suspect was arrested on Saturday after fleeing into the Lapland wilderness on a dog sled.
The police operation involved a helicopter and officers on snow scooters in temperatures of -30C.
The man was said to have been suffering from hypothermia when he was found.
A man who punched and killed his girlfriend just days after he was overheard saying he would kill her has been jailed.
[NEXT_CONCEPT]
An engineering firm is to build a new technology centre in Brechin, creating 100 new jobs.
[NEXT_CONCEPT]
A man has been remanded after appearing in court in Lapland accused of murdering his Scottish girlfriend.