Video shows the bus pulling up at a stop in Xining, in Qinghai province, before the road surface opens up beneath it and the entire vehicle falls inside.
Pedestrians can be seen fleeing as explosions and smoke issue from the 80 sq m sinkhole in the footage broadcast on state media.
“There was water running above, and an explosion went off due to the electricity cable. The sound was huge. A water pipe broke, and water poured in,” CNN reported a witness as having told a government-run magazine.
A man who rescued a baby from the edge of the hole subsequently returned, with others, to help a woman holding on to the edge, CNN reported. However, the road gave way a second time, causing them all to fall.
It happened on Monday outside a health clinic and left six people dead, 16 people in hospital while four are still missing.
In 2018, a woman was swallowed by a sinkhole in Lanzhou City, in Gansu province, which was the fourth to appear in the city that year.
Two years earlier a busy intersection in Hangzhou City, Zheijiang province, was wrecked when a 2m sinkhole opened up – but alert police officers spotted the hazard early and were able to prevent motorists from driving into it.