Hypercalcemia is a clinical emergency which can cause hypercalcemic crisis and even endanger patients' lives. The increase of serum calcium concentration is caused by the redistribution of calcium in bone and the inhibition of parathyroid secretion, which is known as non-parathyroid hypercalcemia. In this report, we presented a rare case of non-parathyroid hypercalcemia during lactation in order to optimize the diagnosis and treatment of this condition. A 27-year-old female patient was admitted to Wuxi People's Hospital on July 11, 2019 due to "fatigue, anorexia, and pain in both knees for half a month". The patient had fatigue and discomfort, accompanied by pain in both knees without obvious inducement. At the same time, the patient had decreased food intake. In the past 3 days, the symptoms worsened, accompanied by limb numbness. The serum calcium level was increased and the parathyroid hormone (PTH) level was decreased. The patient was diagnosed with hypercalcemia, and was treated with calcitonin and lactation termination. The knee pain disappeared and serum calcium returned to normal during a 2-week follow-up. To conclude, the correlation between hypercalcemia and lactation needs to be considered for non-parathyroid hypercalcemia during lactation. After excluding other possible causes, lactation termination therapy may be an effective therapeutic strategy for non-parathyroid hypercalcemia caused by excessive lactation.