The girl, Ren Xin, was born attached to an undeveloped parasitic twin.
Doctors at the military hospital in Beijing where Ren was operated on told the China Daily newspaper that the arm had a shoulder blade, a forearm and two fingers with nails.
Surgery, which took place on Wednesday, was complicated because the girl was fused to the parasite's spine.
Wednesday's operation was only one part of a four-stage program to remove the extra arm. However, doctors are optimistic about the eventual outcome of the operation, which may last a few months.
China's 'angel girl' is not the first child born with a parasitic twin in recent months.
In November, a two-year-old Indian girl born with four arms and four legs underwent a 27-hour operation to remove her extra limbs. The girl was born attached to a parasitic twin, which ceased developing in the mother's womb.
