Using the finite element method (FEM), the mode area, confinement loss and dispersion property of a novel microstructured optical fiber (MOF) with double-cladding structure are numerically simulated, analyzed and compared with those of traditional MOF. Results show that the fiber not only has a novel structure but also has better dispersion performance than the traditional photonic crystal fiber. Through rational optimization, several flat dispersion MOFs with large mode area in the 500 nm wavelength range are designed. The novel fiber proposed will have some reference value to the subsequent theoretical study and manufacture.