The 21 deaths were reported in the worst-affected Wangmo county, where 31 people remained missing, Nong Wenhai, a deputy governor of Wangmo, state run Xinhua quoted officials as saying.
Five people were reported missing in eastern Jiangxi Province.
The number of missing fell from earlier reported 52 as the death toll rose by seven and 14 were confirmed alive as they either visited relatives or worked in cities as migrants and therefore were not at home at the time of the flooding, an official said.
Another person was reported missing in Luodian county.
The floods have forced about 45,400 residents of Wangmo County to evacuate and caused a direct economic loss of 1.34 billion yuan (USD 207 million), the official said.
The floods toppled 801 homes, damaged further 4,813 homes and affected 13,000 hectares of crops and vegetables in Wangmo, he said.
Flood waters inundated the county seat on Monday, disrupting power and water supplies and telecommunication services in several towns.
The flooding also damaged roads, bridges, dikes, irrigation facilities and hydropower stations.
About 6,000 students from four schools had to stay home yesterday, as their schools were rendered inaccessible by the flood waters, said the county's education chief Yuan Tie.
The weather bureau predicts the rainy weather will last until tomorrow.
Heavy rain has lashed several other southern provinces during the past few days.
The local government has mobilised more than 330 soldiers and policemen to clean up roads.
Rain-triggered floods have so far hit 11 cities and counties across Guizhou Province, affecting at least 270,000 people, the provincial civil affairs bureau said.
