Normal bone density is not directly related to whether a tumor has bone metastases, which usually result in decreased bone density. Bone density is an important marker of bone quality and is usually used to reflect the degree of osteoporosis, which can predict the risk of fracture. When bone metastasis occurs in malignant tumors, tumor cells enhance the role of osteoclasts, leading to impaired bone reconstruction and reduced bone mass. Tumor factors and inflammatory factors secreted by tumor cells will enhance the activity of osteoclasts, inhibit bone formation and enhance bone resorption, which will easily lead to osteoporosis. In addition, patients with bone metastasis of tumor, when using some chemotherapeutic drugs, will also increase the bone resorption, reduce the bone formation and lead to the imbalance of bone reconstruction.