State media has quoted military experts as saying the carrier, designed in China and built in the northeast port of Dalian, is not expected to enter service until 2020, as it will take time to fully kit out and arm.
The launch had been well-flagged as foreign military analysts and Chinese media have for months published satellite images, photographs and news stories about the second carrier's development. China confirmed its existence in late 2015.
The carrier's hull is fully constructed, the official Xinhua news agency said, adding that the propulsion, power and other main systems are in place.
The ship's launch "shows our country's indigenous aircraft carrier design and construction has achieved major step by step results," Xinhua said.
State television showed the carrier, its deck lined in red flags, being pushed by tug boats into its berth.
Fan Changlong, a vice chairman of China's powerful Central Military Commission, presided over the launch ceremony, Xinhua said, during which a bottle of champagne was broken on the bow.
The launch follows China's celebration on Sunday of the 68th birthday of the founding of the Chinese navy, and comes amid renewed tensions between North Korea and the United States over Pyongyang's nuclear and missile programmes.
Little is known about China's aircraft carrier programme, which is a state secret.
But the government has said the carrier's design draws on experiences from the country's first aircraft carrier, the Liaoning, bought from Ukraine in 1998 and refitted in China.
The new conventionally powered carrier has a displacement of 50,000 tonnes and will be able to operate China's Shenyang J-15 fighter jets.
China's navy has been taking an increasingly prominent role in recent months, with a rising star admiral taking command, its first aircraft carrier sailing around self-ruled Taiwan and new Chinese warships popping up in far-flung places.
China claims almost all the South China Sea, believed to have huge deposits of oil and gas, through which about $5 trillion in ship-borne trade passes every year, and has been building up military facilities like runways on the islands it controls.
Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have claims.
Self-ruled Taiwan, claimed by Beijing as its own, has said China is actually building two new aircraft carriers, but China has not officially confirmed the existence of another carrier.
Chinese state media has quoted experts as saying that the country needs at least six carriers, and a network of bases around the world to support their operations, though the government has been coy about suggestions it wants a global military presence to match the United States.
The Liaoning has taken part in military exercises, including in the South China Sea and more recently near Taiwan, but is expected to serve more as a training vessel than having an actual combat role.
The People's Liberation Army Navy significantly lags behind the US, which operates 10 aircraft carriers.
