Hu Jingxuan's condition deteriorated over the weekend, and she died Monday afternoon, said an official at Fudan University Children's Hospital. The official, who works in the hospital's administration office, refused to give her name, saying she was not officially authorized to speak to media.
Chen Jingni, the stronger of the twins, was in stable condition, the official Xinhua News Agency reported.
Doctors at the hospital had branded Thursday's separation surgery a "miracle" but had cautioned that the girls, who also suffered from congenital heart disease and other problems, were in precarious conditions.
Doctors said they were forced to operate because the girls had stopped gaining weight.
The babies, who were born to a farm family, shared a liver, spleen, gall bladder and digestive tract before the 13-hour-long surgery.
The twins were born weighing a total of 10 pounds. Before surgery, they weighed 16 pounds, the hospital said. They looked thin and frail in photos provided by the hospital.
The hospital, which has conducted numerous separation operations, began planning the girls' surgery right after their birth. A business group in Taizhou, the twins' hometown, raised more than $25,000 to help pay for their treatment, reports said.