Abstract: The effects of refractive index and fiber core structure on energy distribution were studied by setting up laser transmission model in single mode fiber with Comsol and Matlab. First, the relationship between the fiber core, numerical aperture, normalized frequency and power fill factor was analyzed. Second, the energy density distribution of various designs for the core refractive index of single mode fiber was numerically simulated and compared. According to the calculation and the simulation results, it is concluded that central-dip fiber can enlarge mode area and lower the power density. Finally, due to the application requirement of high power fiber laser, the single mode central-dip fiber was designed with the wavelength of 1.064 m, the core diameter of 10 m, the concave diameter of 8 m, and the numerical aperture of 0.12, which provides an idea for the development of pump efficiency and the reduction of the energy density in fiber core.