The 50-year-old woman, identified only as Lisa, had registered in the Wolong Giant Panda Protection and Research Center in Sichuan province as a volunteer, according to the official Xinhua News Agency.
She was wearing gloves and feeding the panda bamboo on Tuesday morning when "suddenly, the panda bit into her thumb," Xinhua said.
"When she cried out, the cub became excited and gripped more tightly," it said. "Lisa finally managed to wrench herself free."
About 20 percent of the thumb had been bitten off, Xinhua said.
A man who answered the telephone at the center's administrative office said it had no comment.
The center has been recruiting volunteers since 2004 for stints that last from one day to more than a month, Zhang Hemin, the center's director, was quoted as saying. They can prepare food, feed pandas under supervision and clear the enclosure with the help of professional keepers, he said.
More than 1,200 volunteers from America, Japan, Switzerland, France and Spain already have worked in the center, Xinhua said.
Last month, a drunken Chinese tourist bit a panda at the Beijing Zoo after the animal attacked him when he jumped into the enclosure and tried to hug it.