The man asked her if she could help fasten the baby seat in his car.
She went over to the car on 16th Street and started pulling on the seatbelt when the man kicked her in the back and then pushed her in the backseat.
The baby in the car seat, police say, was a doll.
"This male then locks the backdoor. He immediately gets behind the wheel of the vehicle," Captain John Darby of the Philadelphia Police said.
He began driving northbound on 16th Street.
The suspect then threatens the frightened 20-year-old.
"He displays, but doesn't hold in his hand a handgun. She observes a black handgun. He threatens her if she screams or tries to do anything he's going to shoot her," Darby said.
A block away, the suspect stops at a red light at 16th and Arch Streets..
The student unlocked the door and ran from the car.
Police say the man did have a fake baby in the car seat.
The suspect is described as a black male, mid -20s, around 5 foot tall with black hair, dark eyes, a full beard, a medium complexion, and was wearing a black t-shirt with silver writing on the front and blue jeans.
He was driving a black car with four doors and gray seats.
Anyone with information should call Special Victims Unit at 215-685-3253 or 54.