It is the country's second aircraft carrier, after the Liaoning, and the first to be made domestically.
The as-yet unnamed ship was transferred into the water in the north-eastern port of Dalian, state media said. It will reportedly be operational by 2020.
It comes amid heated rhetoric between the US and North Korea and ongoing tensions in the South China Sea.
China has had only one operational aircraft carrier, the Liaoning, which it bought from Ukraine and refitted.
The US has deployed warships and a submarine to the Korean peninsula, prompting an angry reaction from North Korea. China has urged calm.
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A big step for Beijing, by the BBC's China Correspondent Stephen McDonell
The sight of a bottle of champagne hitting the bow of a new Chinese-built aircraft carrier will worry many.
Western military intelligence will be poring over the television footage. For them it's not emotional: cold calculations are being made.
They see a not quite finished vessel, a few years from full service, partly based on Soviet-era design.
It's technologically inferior to the ten aircraft carriers being used by the United States Navy - but there's no doubt that it's a big step for China.
China's aircraft carrier programme is a state secret, but it's hard to imagine this country being satisfied with two of them.
The US says all options are on the table to remove North Korea's nuclear weapons - and it is using the USS Carl Vinson battle group to press the point.
That's the kind of power that China wants - and that's why we haven't seen the last Chinese aircraft carrier rolling off the production line in the Dalian shipyard.
BBC