We propose a graphene-embedded waveguide (GEW) with improved modulation capability, 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.