The incident has sparked outrage on China's hugely popular social media sites.
Surveillance cameras showed a series of people walk past the girl, named Yue Yue, after she was hit first by a van and then a truck outside her family's shop in the southern Chinese city of Foshan.
Xinhua said a rubbish collector who finally came to the girl's aid, moving her to the curb and shouting for help, was ignored by several shopkeepers before he finally tracked down her mother who took her to hospital.
In response, one netizen on Sina Weibo, a Chinese micro-blog similar to Twitter, wrote: "This society is seriously ill. Even cats and dogs shouldn't be treated so heartlessly."
But others linked the incident to an earlier case in which a man who tried to help an elderly woman after she fell over was prosecuted, apparently because his intervention broke government rules on dealing with accident victims.
Doctors said Yue Yue was in a coma and unlikely to survive the ordeal.
"She would not be able to survive any operations. She's very close to brain death," a spokesman for the hospital treating her told reporters.
Police have detained the drivers of both vehicles involved in the incident, Xinhua said.