Ai, who was held in a secret location for 81 days as part of a widespread crackdown on rights activism in China earlier this year, says the charge is politically motivated and has vowed to challenge it.
On Tuesday the 54-year-old, facing a bill for 15 million yuan ($2.4 million) in alleged back taxes, handed 8.45 million yuan in donations from his supporters to the Chinese authorities as a bond to clear the way for an appeal.
"We can use this as a chance to make the world understand what kind of system they are working with," Ai said in a telephone interview with AFP as he prepared to challenge the charge, calling it a "real opportunity".
"We just paid 8 million only because I want to have freedom of speech," he said, adding that the sums involved were unimportant "compared to the kind of social achievement it will have".
Ai said he had signed a contract with tax authorities on Wednesday agreeing that the $1.3 million in donations from his supporters he and his lawyers handed over on Tuesday would be used as a bond to clear the way for an appeal.
The money was raised from supporters who came from far and wide to help him raise cash, some even throwing money over the walls into his courtyard home, including banknotes folded into paper planes.
Total donations had reached 8.69 million yuan ($1.4 million) by Sunday night, when the appeal closed, and Ai said the generosity of the Chinese people had made him realise that he was "not alone" in his struggle.
"So far, I am still in very good spirits because all the money has come from donations," Ai told AFP by telephone as he headed to the tax office to sign an agreement that the money can be used as collateral by the authorities.
Chinese media have devoted little coverage to the case, but on Wednesday the state-run Global Times questioned whether there was much domestic support for Ai, comparing him to 1970s democracy activist Wei Jingsheng.
Wei now lives in exile in the United States and is all but forgotten in China.
In response, Ai said "at least 95 percent" of the money sent to him was from inside China.
Chinese tax authorities have repeatedly refused AFP's requests for comment on Ai's case, which is particularly complicated because the company involved in the tax evasion charge is owned by his wife and not him.
He has said he will pay back his supporters once the case is over, and was initially reluctant to hand over the money raised to Chinese authorities for fear that it would not be returned to him.
Ai is known for his often irreverent art and for tallying the children killed in the 2008 Sichuan earthquake, shining an unflattering light on officials who some accused of covering up the role shoddy housing played in the deaths.
The value of his works has shot up since his detention in February catapulted him into the global spotlight, and last month the influential Art Review magazine named him the world's most powerful figure in the art world.
Ai, who last year covered the floor of London's Tate Modern museum with 100 million porcelain sunflower seeds as part of an exhibition, became only the second artist after Damien Hirst to top the ten-year-old list.
A prolific user of Twitter and other social media, he told AFP on Wednesday he currently had little time for his regular work, but that he considered his fight against the government to be an extension of his artistic endeavours.
"I think this is my artwork. My artwork is about communication and expressing my social concern," he said.