Despite being diverted through 13 giant sluices into an adjacent buffer zone and designated low-lying fields, the Huai remains threateningly high, Xinhua said.
"The three provincial authorities have mobilised 511,000 people to patrol the Huai embankments and issue emergency warnings," the state flood control and drought relief headquarters was quoted as saying.
On Wednesday, water levels reached 29.55 metres, a full 25cm above the danger level.
China's leaders urged local governments to improve weather monitoring, and to prioritise public safety by relocating flood victims.
Millions displaced
Tens of millions of residents across southwestern, central and eastern China could be affected by the worsening floods.
Continuous torrential rains threaten to inundate
central and eastern China [GALLO/GETTY]
At least 360 people have been killed this summer in the floods and related disasters which have destroyed more than four million hectares of crops.
Xinhua said the state flood control and drought relief headquarters had estimated the direct economic losses at 24.3 billion yuan ($3.13bn).
The Huai flows through densely-populated farmland between China's two major rivers, the Yellow and the Yangtze.
Its basin contains a complex system of lakes, tributaries, and canals, which carry water at a sluggish pace to the Yangtze and the Pacific Ocean.
Various bottlenecks and elevation changes make flooding a regular occurrence during the summer rainy season.
Meanwhile, droughts in northern China have left at least 7.4m people short of drinking water, Xinhua said.
China is also trying to deal with two billion flood-displaced rats in the eastern province of Hunan alone which have destroyed 1.6 million hectares of crops.