Answer by taking a quote from the following article:

Ai Weiwei (Chinese: Ai Wei Wei ; pinyin: Ai Weiwei,  English pronunciation ; born 28 August 1957 in Beijing) is a Chinese contemporary artist and activist. His father's (Ai Qing) original surname was written Jiang (Jiang ). Ai collaborated with Swiss architects Herzog & de Meuron as the artistic consultant on the Beijing National Stadium for the 2008 Olympics.

On 22 June 2011, the Chinese authorities released Ai from jail after almost three months' detention on charges of tax evasion. Beijing Fa Ke Cultural Development Ltd. (Chinese: Bei Jing Fa Ke Wen Hua Gong Si ), a company Ai controlled, had allegedly evaded taxes and intentionally destroyed accounting documents. State media also reports that Ai was granted bail on account of Ai's "good attitude in confessing his crimes", willingness to pay back taxes, and his chronic illnesses. According to the Chinese Foreign Ministry, he is prohibited from leaving Beijing without permission for one year. Ai's supporters widely viewed his detention as retaliation for his vocal criticism of the government. On 23 June 2011, professor Wang Yujin of China University of Political Science and Law stated that the release of Ai on bail shows that the Chinese government could not find any solid evidence of Ai's alleged "economic crime". On 24 June 2011, Ai told a Radio Free Asia reporter that he was thankful for the support of the Hong Kong public, and praised Hong Kong's conscious society. Ai also mentioned that his detention by the Chinese regime was hellish (Chinese: Jiu Si Yi Sheng ), and stressed that he is forbidden to say too much to reporters.  After his release, his sister gave some details about his detention condition to the press, explaining that he was subjected to a kind of psychological torture: he was detained in a tiny room with constant light, and two guards were set very close to him at all times, and watched him constantly. In November, Chinese authorities were again investigating Ai and his associates, this time under the charge of spreading pornography. Lu was subsequently questioned by police, and released after several hours though the exact charges remain unclear. In January 2012, in its International Review issue Art in America magazine featured an interview with Ai Weiwei at his home in China. J.J. Camille (the pen name of a Chinese-born writer living in New York), "neither a journalist nor an activist but simply an art lover who wanted to talk to him" had travelled to Beijing the previous September to conduct the interview and to write about his visit to "China's most famous dissident artist" for the magazine.  On 21 June 2012, Ai's bail was lifted. Although he is allowed to leave Beijing, the police informed him that he is still prohibited from traveling to other countries because he is "suspected of other crimes," including pornography, bigamy and illicit exchange of foreign currency. Until 2015, he remained under heavy surveillance and restrictions of movement, but continues to criticize through his work. In July 2015, he was given a passport and may travel abroad.

what was he charged with?
Ai's supporters widely viewed his detention as retaliation for his vocal criticism of the government.