The victim, Adaisha Miller, was shot at the officer's home early Sunday, WDIV-TV, Detroit, reported.
Police officials said Miller embraced the officer, whose name had not been released, from behind, causing his holstered weapon to go off.
Miller, who would have celebrated her 25th birthday Monday, was hit in the lung and heart and died later at a local hospital.
Yolanda McNair, Miller's mother, said she does not understand how the accident occurred, the Detroit Free Press reported.
McNair said she was initially told her daughter "was being a little flirty" with the officer and the gun fired as they were hugging face-to-face.
"The story keeps changing," McNair said. "There's no logical reason."
McNair said she wondered why an off-duty officer at a party would "need a weapon with a round in the chamber."
John Goldpaugh, a lawyer for the Detroit Police Officers Association, said it was "just a freak accident."
"They were having a party and the next thing a woman is dead," Goldpaugh said. "He's devastated by what happened."
The officer will be assigned to administrative duties during an investigation, police spokeswoman Sgt. Eren Stephens said.