In order to expand the low reflective properties of silicon solar cells in a perfect frequency range, a kind of optical microstructures composed of period arranged square pillar was designed. On the base of the effective medium theory, the reflectivity of silicon solar cells changing with the pillar′s altitude and duty cycle was studied. Then, a structural model was set by simulation software "MEEP". The results of the simulated research demonstrate that the optical microstructures can very effectively reduce the reflection in a perfect optical frequency range.