Heavy rains also closed Beijing's airport, stranding almost 10,000 passengers as nearly every departing flight was cancelled Monday night, Xinhua News Agency said.
Gates on the huge Three Gorges Dam in central Hubei province were opened to allow water to flow out, Xinhua said, but heavy rains meant inflows would be greater with another peak expected.
"The Three Gorges Dam has opened 18 sluices and the water level in the reservoir will continue to rise," a worker in the dam's operation department was quoted saying.
"The safety of the dam will be tested."
Two-way traffic at the Three Gorges ship lock was halted Monday because of the water levels.
The Three Gorges dam, the world's biggest hydropower project, was promoted by the government as a way to control devastating flooding on the Yangtze River and as a clean power source.
Besides the 27 killed, Xinhua also said 24 were missing after more rainstorms hit numerous parts of China. That included 16 dead and 14 missing in northwestern Shaanxi province after downpours hit the central and southern areas of the province, it said.
The other deaths were in the Inner Mongolia region and in Hubei, Guizhou, Shanxi and Anhui provinces.
The 27 deaths come after Xinhua reported floods, landslides and mud flows triggered by torrential rains had killed 652 people in China so far this year.