Ai is an outspoken critic of the Chinese government, which has responded by harassing him in various ways. In April 2011 authorities seized the passport as Ai was on his way to Hong Kong, and detained him for 81 days for alleged tax evasion and financial misconduct.
He was released on probation for those charges, then later was charged with trafficking in pornography after exhibiting photographs of himself in the nude. Security police also closely monitor his movements within Beijing, where he is essentially under house arrest.
Plus, his studios outside Shanghai were bulldozed down, supposedly because he had failed to get proper permits and pay taxes on the property.
This week, also for unspecified reasons, authorities gave him a new passport. Ai posted a photo of it in his Instagram account @aww.
This is China.
More details at CNN.