The incidence and mortality rate of malignant tumors in China are increasing year by year. The occurrence and development of malignant tumors, especially gastrointestinal tumors, are closely related to disorder of intestinal microenvironment. Fatty acids, especially essential fatty acids, are important nutrients for human growth and life support. Fatty acids are actively involved in the intestinal microbiota during digestion and absorption in the gastrointestinal tract. Disorders of fat metabolism are closely related to the intestinal microbiota, and are associated with gastrointestinal inflammation, increased permeability of intestinal wall and the entry of toxins into the body, to resulted in an immune response. Intestinal microbiota, through cross-reactions between pathogenic microorganisms and tumor antigens, form a T-cell reserve and/or microbial product that stimulates pattern recognition receptors, affect the type and intensity of the immune response. Through the effects on immunity, microbiota help to control of or escape from the immune response to distant tumors. Microbial-derived metabolites can also promote the development of local tumors through direct cell-independent carcinogenesis. There is a relationship between the intestinal microbiota-gut-brain axis or the microbial-intestinal-liver axis with the external organs of the gastrointestinal tract. The important role of fatty acid nutrition and intestinal microenvironment should be taken into account in the prevention and treatment of malignant tumor. Fatty acids may also be given through the parenteral pathway, depending on the patient's needs. Patients with advanced malignant tumors are often accompanied by malnourishment or heterogeneity. Supplementation of ω -3 polyunsaturated fatty acids can improve the physique of patients and contribute to the treatment of tumors. By supplementing probiotics or transplanting intestinal microbiota, the disorders of intestinal micriobiota can be fixed and play an important role in the prevention and treatment of tumor.