A graphene-embedded waveguide （GEW） with improved modulation capability was proposed， which can be over 2 times larger than that of conventional graphene-on-silicon （GOS） waveguide. More importantly， it is found that the improvement of modulation capability mainly results from the enhanced electric field confinement around graphene. Based on this finding， we propose a high-efficient method to optimize the modulation capability. By using this method， the optimization work can be reduced by an order of magnitude. Our work may promote the design of graphene-based electro-optic modulator with high modulation capability.