Obesity is a global health issue closely related to various digestive system tumors. Studies have shown that obesity is associated with digestive system tumors such as esophageal adenocarcinoma, gastric cancer, colorectal cancer, hepatocellular carcinoma, biliary tract cancer, and pancreatic cancer. This article elaborates on how obesity significantly increases the risk of digestive tract tumors (such as esophageal adenocarcinoma, gastric cancer, and colorectal cancer) through mechanisms such as metabolic disorders, chronic inflammation, insulin resistance, and abnormal sex hormone levels. The risk increases with the degree of obesity. Inflammatory factors secreted by fat cells can activate pro-inflammatory signaling pathways and promote tumor cell proliferation; insulin resistance leads to hyperinsulinemia, activates signaling pathways, acidifies the tumor microenvironment, and facilitates tumor invasion and metastasis; abnormal sex hormone levels activate estrogen receptors and promote tumor cell growth. In addition, obesity-induced tumor-associated macrophages (TAMs) and cancer-associated adipocytes (CAAs) further promote tumor development by secreting pro-inflammatory factors and providing energy. Future research will focus on the causal relationship between obesity and tumors, and develop drugs targeting related targets (such as GPR65, PHD3, Glce) to assist in obesity prevention and control and tumor treatment.