Ai was released in June after 81 days in police custody when he was accused of tax evasion, although he said he was repeatedly interrogated over his calls for human rights.
"They gave a written notice today ... there was no explanation whatsoever. We questioned where this figure came from. They couldn't give a clear answer," Ai told the AFP news agency.
"The notice said I have 15 days to pay. That's about one million a day ... if you don't pay they could put you in jail, maybe up to seven years. I really have no idea."
Ai, 54, a globally renowned conceptual artist who is famous for his part in designing Beijing's "Bird's Nest" Olympic Stadium, said he may be able to appeal against the tax bill, but only after it is paid.
His mother has offered to sell the house left to the family by Ai's father, the late Ai Qing, a well known poet praised by the Communist Party, but who also suffered political persecution.
Ai called the tax an "injustice" and alleged it was a political move by the government to silence him.
Ai said the tax was being charged to him as "the actual controller" of the Beijing Fake Cultural Development Ltd, a company where he works but which is owned by his wife.
Ai’s detention sparked international protests.
After his release, Ai was warned not to discuss his case publicly or accept interviews from foreign journalists.