"A major flood is feared if rain continues," Huang Boqing, deputy director of flood control in the southern province of Guangdong, was quoted as saying.
In earthquake-hit Sichuan province, some 70,000 people were reported to have been evacuated from the town of Wenchuan, close to the epicentre of last month's earthquake.
Most have been living in temporary tent cities after their homes were destroyed in the quake.
|More rains are forecast for |
the coming days
Experts warned of an increased risk of mudslides in the area caused by heavy rains hitting ground already loosened by the quake and hundreds of aftershocks.
Wenchuan was virtually flattened by the May 12 quake that left up to 87,000 dead or missing and up to five million homeless.
Elsewhere in the country, officials have been rushing to shore-up flood defences along some of China's major rivers.
The situation on the Pearl river in southern China's Guangdong and Guangxi provinces was said to be the most dangerous with water levels at a 20-year high.
Water levels were reported to have surpassed warning levels by almost seven metres on one tributary of the Pearl in Guangxi.
Hardest hit has been Guangdong province – a manufacturing area that borders Hong Kong.
The official Xinhua news agency said at least 20 people had died in the province with eight others listed as missing.
The flooding has also driven up prices in local markets, with vegetable prices in Guangdong's biggest cities rising by around 70 per cent.
The state-run China Daily newspaper said the flooding of towns in the Pearl River Delta region was the worst in 50 years.