Disclosed herein is a fuel injection system, including: a fuel injection module including a primary fuel injector and one or more secondary fuel injectors disposed around the primary fuel injector; an air supply module for supplying air to the fuel injection module inwardly and outwardly; and a fuel supply module for supplying fuel to the fuel injection module, wherein the fuel injection module serves to generate multistage flames in a burner by forming a rich fuel flame region using the primary fuel injector and forming a lean fuel flame region behind the rich fuel flame region using the secondary fuel injectors through a burning process of gasifying secondary fuel.