The wave-guide structure of the large mode area fiber with a comb-index core was optimized by adding a high refractive index platform in the outer layer of the fiber core, which provided a performance promotion on the parameters of mode field area, mode field distribution and bending resistance. The simulation results show that the optimized refractive index distribution increases the fiber′s mode field area by more than 700 μm2, and reduces bending loss of the fundamental mode from 6 dB/m to 0.1 dB/m at the same bending radius compared with the original comb-index distribution. Moreover, the mode field distributions including Gauss-like distribution, flattened distribution and hollow distribution, can be obtained by simply modulating the refractive-index parameters of the outermost platform, which may meet the needs of special laser processing industry.