The province of Gansu battled severe storms on Thursday, with at least 100 millimetres of precipitation recorded in Linxia Hui Autonomous Prefecture, one of the worst-hit areas.
Dozens of vehicles were washed away by floodwaters as helpless residents watched on.
The flood occurred after torrential rain pounded the provincial capital of Gansu overnight.
At least 12 people have died and another 39 were injured in flash floods, according to reports citing local authorities. Four people remain missing and over one million people were affected across the province.
“Due to continuous rainfall, vehicles parked on both sides of the road in Lanzhou city were washed away by floods – just like toy cars,” the China Weather Network said in a statement on Friday.
Crops, livestock and infrastructure such as bridges and roads were also washed away or destroyed.
In Cuijia Village in Dongxiang, heavy rain caused a deep ditch, splitting the village in two.
Armed police are searching for people from a village livestock farm, which was washed away Thursday morning. Many houses and infrastructure in Chenhe Village have also collapsed.
State television said 241 rivers in 24 provinces had flooded in the past week, causing direct economic losses of 25.9 billion yuan (£2.9 billion), according to Reuters.