Abstract: With the increase of obese people around the world, the complications associated with obesity are endangering human health, it has became an important factor affecting the quality of life and increasing social and economic burdens. Roux-en-Y gastric bypass (RYGB) is an effective surgical tool for the treatment of obesity and metabolic related complications, meanwhile, it can also improve or cure the obesity related metabolic diseases with reducing the body mass of patients. Free fat mass (FFM) is the main component of the human body and plays an important role in metabolic surgery for the prevention of bone loss, control of weight and alleviation of insulin resistance and so on. The loss of body mass is one of the manifestations of the changes in body composition and distribution after RYGB, and its performance is common after metabolic surgery. This article will summary the findings and investigate the reason of decreased FFM after RYGB and the preventive strategy for different reasons, and it will also provide to the principle of individualized management of patients and guide the clinical practice.