More than 1.6 million people have been forced from their homes, the civil affairs ministry said.
Rising water levels
More than 750 government officials and police have been posted at six reservoirs in "danger of bursting" in the southern Guangxi region, the official Xinhua news agency said.
The swollen Xijiang and Beijiang rivers in Guangdong were subsiding slowly but rain forecast over the next three days would provide renewed risk of flooding, it said.
Heavy rains forecast for neighbouring Fujian province could also "cause geological disasters", added Xinhua.
The national meteorological bureau forecast storms in western Guangdong and southern Guangxi, and warned authorities to stop outdoor work and guard against damaged electric cables.
"Be on guard for disasters including mountain flooding, landslides and mudslides," the bureau said on its website.
Chinese officials are also worried about flooding on the Yellow River in the north, the country's second-longest waterway which flows through some of the most populous and poorest provinces.
Provincial water authorities earlier reported the Pearl River Delta, a major exporting base in Guangdong province, had suffered its greatest flooding in 50 years.
The massive floods have toppled more than 130,000 houses, damaged or destroyed more than two million hectares of farmland and caused an estimated $4bn in economic losses.