Epidemiology has found that diabetes mellitus is related to the occurrence and development of autoimmune diseases, tumors and other diseases, and that diabetic people are more prone to such diseases as tumors, and have more severe symptoms and worse prognosis than non-diabetic people. Colorectal cancer (CRC) is a common malignant tumor in the digestive system and one of the main cancers that threaten the life and health of Chinese and cause a decline in their quality of life. Relationship between diabetes, type 2 diabetes mellitus (T2DM) in particular, and CRC has drew increasing attention in recent years. The biggest concern by scholars is that T2DM can be an independent influencing factor of CRC, and this may be related to insulin resistance, hyperglycemia, inflammation, and exogenous insulin. This article reviews the correlation between T2DM and colorectal cancer and the potential mechanisms.