More than 10,000 people in Henan province have been evacuated to shelters following the record rainfall.
At least 12 people have died in the city of Zhengzhou since the flooding began, authorities there confirmed.
More than a dozen cities have been affected with main roads forced to close and flights cancelled.
Henan province, which is home to some 94 million people, has issued its highest level of weather warning following an unusually active rainy season.
Many factors contribute to flooding, but a warming atmosphere caused by climate change makes extreme rainfall more likely.
Images on social media showed entire streets submerged, with cars and debris left drifting in the fast-moving floodwaters.
There are also fears that a dam in Henan province could collapse after it was damaged by the recent storms.
A 20-metre (65ft) breach has emerged in the dam in Luoyang city, officials said. Soldiers have been deployed to the area and a statement from the army warned it could "collapse at any time".
In Zhengzhou, unverified video footage showed passengers in flooded subway carriages with water reaching their shoulders.