The artist, who created the Sunflower Seeds installation at the Tate Modern museum in London, said officials told him "my acts might affect national security."
He said he believes police were concerned that he would attend the Dec. 10 Nobel Peace Prize ceremony for jailed Chinese dissident Liu Xiaobo, even though he told officials he would not attend.
Activists, friends and supporters of the imprisoned writer have been placed under house arrest, subjected to surveillance or barred from international travel since the Nobel prize announcement.
Ai told the British newspaper The Guardian he was at the boarding gate to fly to Seoul Friday when a border official told him a "technical problem" occurred and she needed to see his passport.
"I said, 'Please tell me the truth -- you are preventing me from going abroad.' She laughed and said 'Yes'," Ai said. "They gave me a written note saying my acts might affect national security so they were stopping me from going out."
The artist has run afoul of authorities for his public criticism of the government and political activism, The Guardian said. In November, he was placed under house arrest after planning an event to mark the forced demolition of his studio in Shanghai.