Rebecca Simpson was pronounced dead after being found at her home on Smawthorne Grove, Castleford.
Ricky Knott, 32, is on trial at Leeds Crown Court accused of murdering 30-year-old Miss Simpson on August 26 last year.
The jury was told neighbours found Miss Simpson unconscious at the bottom of the stairs after being alerted by Knott.
Simon Myerson QC, prosecuting, said an ambulance was called to the property shortly after midnight but Miss Simpson was pronounced dead in hospital later that day.
Mr Myerson said Miss Simpson died from a major blunt force injury to the back of the head.
Miss Simpson and Knott had been in a relationship for seven months and had been out drinking with friends earlier in the evening.
The couple returned to Miss Simpson's home where they planned to spend the night together.
Knott was arrested after Miss Simpson's death but did not answer questions when interviewed.
He instead gave written statements after each interview and claimed his girlfriend's death was a "tragic accident."
Mr Myerson told the jury of six men and six women: "He said he was the only person in the house apart from Rebecca Simpson when Rebecca Simpson fell down the stairs.
"He has no previous convictions. He has never been in trouble with the courts.
"He says that he went upstairs and Miss Simpson was going to follow him upstairs.
"They had had a good evening out. There were both drunk but there were no arguments.
"He was in, or almost in the bedroom when he heard the thud. When he looked he saw Miss Simpson at the bottom of the stairs."
Knott went on to state that he panicked as he could not find his mobile phone and went to alert neighbours who called for an ambulance.
The defendant also stated in his account that Miss Simpson may have fallen because she was drunk or her knee may have given way as she was awaiting surgery for damaged knee ligaments.
Mr Myerson added: "The prosecution say most of what he says is untrue and demonstrably untrue."
The court heard Miss Simpson suffered other injuries including a gash to the front of her head which Knott could not account for.
Jurors also heard that blood was found in other parts of the house.
Blood was found near to the front door, including blood spattering above the skirting board.
The prosecutor said: "After hearing evidence, we would say to you that Rebecca Simpson was plainly injured before she fell down the stairs.
"The only person who could have inflicted the injuries was the defendant.
"If this had been an accident he would have told us everything."
He added: "This was no accident."
The court heard Knott told paramedics that Miss Simpson had taken cocaine during their night out.
Mr Myerson said Miss Simpson's body was tested for drugs and there was no trace of cocaine.
He said: "You will have to consider whether that was another lie or whether it was a simple mistake."
The prosecutor continued: "When the paramedics asked if he needed a jacket he said 'why?'
"They assumed he would be coming to the hospital with them. He said 'do I have to?'
"You will say whether that is a forgivable and understandable action of a man who was shocked because he had seen his girlfriend at the bottom (of the stairs) or whether it reflects something of his attitude towards her that night."
Knott, of Arnside Close, Castleford, pleads not guilty to murder. The trial is expected to last two weeks.