It joins another 10,000-tonne Chinese vessel, which was reportedly completed last year and patrols the East China Sea, as the biggest coast guard patrol ships in the world.
The move looks set to increase China's presence in the contested waters of the South China Sea, just days after a senior retired People's Liberation Army (PLA) official said that the Chinese military will probably conduct test flights soon from a controversial airport in the disputed Spratly Islands there.
This comes as Japan has decided that its P-3C patrol aircraft returning home from anti-piracy activities off the Somalian coast will give priority to stopping at bases of countries facing the South China Sea, including the Philippines and Vietnam, the Yomiuri Shimbun reported.
The completion and planned deployment of China's second "monster" coast guard vessel - as dubbed by the media - will be a "cornerstone of the country's maritime enforcement strength", reports said.
Beijing-based news website Guancha said the vessel will have a displacement of 12,000 tonnes and a top speed of 25 knots. It added that the ship will be fitted with a 76mm quick-firing main gun, two secondary guns and two anti-aircraft guns.
The "China Coast Guard (CCG) 3901" ship will also have a helicopter pad. The ship is similar to the design of the CCG 2901, which was deployed last year in the East China Sea, where China has a territorial dispute with Japan.
Military expert Song Zhongping said China needs such a large vessel, given its purported objective of patrolling the South China Sea, which stretches for 2,000km and is bigger than the East China Sea. A larger ship can be out at sea longer, hold a bigger crew and carry more supplies. It is also more effective if a clash happens with other ships. "Given the distance and resources that are needed for such a vast area, it's no surprise China built a ship of this size. It's necessary," Mr Song said.
The previous biggest CCG patrol ships measured only 4,000 tonnes.
The "monster" vessels are part of the CCG's robust shipbuilding plans. Experts have said that China is quickly expanding its coast guard fleet as part of its plans to strengthen its naval capability. But this comes amid rising tensions in the South China Sea as China is seen to be increasingly assertive in its territorial claims there that overlap with those of Asean states Vietnam, the Philippines, Malaysia and Brunei.
Retired PLA major-general Xu Guangyu said last Friday that a controversial new Chinese airport built on Fiery Cross Reef will likely be used by Chinese military aircraft for test flights by June.
Chinese civilian aircraft had just last week completed two test flights at the airport.
"Military aircraft will take off from there sooner or later. Very likely within the first half of the year," he told the Hong Kong newspaper South China Morning Post.
The PLA last month also added one of its most advanced missile destroyers to its South Sea fleet, which operates in the South China Sea. That is the third advanced destroyer to be deployed there in the last two years.
The expansion of the CCG's ships is likely to continue, experts say.
"One 10,000-tonne coast guard vessel isn't enough for the South China Sea," said Mr Song. "We can expect more of them to be built."