The accident happened on Saturday at a park in Mengcheng county in the eastern province of Anhui, the Shanghai Daiy said.
"Park workers managed to pull the boy away, but his left arm had been torn away by the animal," the paper said.
The boy was rushed to hospital and underwent surgery, the paper said, without giving details of his condition.
Police said the space between the bars of the cage was too wide.
Patchy adherence to safety standards and lax supervision have led to a number of children being killed or harmed by
animals at Chinese zoos in recent years.
Sixteen government officials were sacked for negligence after a nine-year-old boy was eaten alive at a crocodile enclosure in the southern region of Guangxi in April.
Last February, a tiger attacked a six-year-old girl waiting to have her picture taken with the animal at a zoo in southwestern Yunnan province, biting her head and killing her..