"I thought the plane was going to explode ... It looked like passengers next to the open door were fainting," an unidentified 44-year-old passenger told the Yonhap News Agency.
The passenger said the cabin crew had made an in-flight announcement asking if there were any doctors on board.
A video aired on television, reported to have been shot by a passenger, showed the moments before the landing, with a door open and wind rushing in as passengers sat nearby.
The transport ministry said in a statement police had taken into custody the man who had opened the door and authorities were investigating violations of aviation safety laws.
The ministry did not say why he had opened the door.
A transport ministry official told Reuters authorities were looking into whether Asiana Airlines had followed protocols to manage emergency exits.
The official said it was possible an emergency exit could be disarmed near the ground when pressure inside and outside the cabin were similar.
The plane was two or three minutes away from landing when the male passenger sitting next to an emergency exit opened a cover and pulled a lever so the door opened about 200 metres (656 ft) above the ground, an Asiana spokesperson said.
All onboard were seated with seat belts fastened because the plane was about to land, the spokesperson said.
