As of late Wednesday, more than 470,000 people and over 55,000 hectares of crops have been affected by the Xinxiang downpours, Xinhua said, adding the local government had deployed a more than 76,000-strong search and rescue team.
In neighbouring Hebei, two people were killed when a tornado struck the city of Baoding.
The fatal flooding of the Zhengzhou subway prompted the government to order local authorities to immediately improve urban transit flood controls and emergency responses.
Media images showed commuters immersed in chest-deep waters in lightless cabins. One underground station was reduced to a large churning pool.