With the improvement of living standard in our country, the diet structure of our country’s residents has changed significantly. Due to the excessive intake of fatty foods and less time for exercise, the incidence of fatty liver is increasing year by year. We have previously studied together the clinical manifestations, diagnostic methods, and treatment principles of fatty liver. Today we are going to discuss how fatty liver is caused. We all know that carbohydrates, fats and proteins are the three main sources of energy for the human body. The human body consumes energy gradually as people’s activities and metabolism. It is possible to achieve a state of balance between energy intake and energy consumption. If people consume too much energy for a long time, especially fatty tissues, and if there is not enough energy consumption, it will cause a large amount of fatty tissues to accumulate in the body. Excessive accumulation of fatty tissues in the cells of the liver can cause edema and degeneration of mitochondria, lysosomes and other organelles in liver cells, which can lead to liver cell damage and even necrosis. Once there is more fatty tissue accumulation in the liver cells to form a fatty liver, causing damage to the liver cells, it can be shown under ultrasound examination as fine and dense enhancement within the liver cells and increased echogenicity of liver tissue. Blood tests may show a mild increase in liver function transaminases and bilirubin, and CT tests may show that the density of the liver is significantly lower than that of the spleen. If patients in the early stage of fatty liver can improve their dietary habits and structure and strengthen exercise, they can avoid progressive aggravation of liver damage and achieve better treatment results.