The tunnel is part a new underground railway system which will be ready in 2011.
The collapse left a crater 15 metres deep and 75 metres wide, which several vehicles plunged into. A passenger in a taxi was killed.
Those still missing are believed to be construction workers.
The construction company has sent workers to reinforce remaining parts of the tunnel, but the government did not rule out the possibility of further collapses.
Houses around the accident site have been evacuated.
The accident was not the first in the history of China's underground railway systems.
Last year, four construction workers were killed when a subway line built for this year's Beijing Olympics caved in.
Two months later, another accident at a subway construction site in the eastern city of Nanjing killed two workers.
In recent years accidents at bridges, schools, building sites, factories and mines have claimed hundreds of lives.