In this work, a bidirectional grating coupler for perfectly vertical coupling with enhanced coupling efficiency is proposed. A silicon nitride layer above the grating regain is utilized to enhance the coupling efficiency. With the help of the silicon nitride layer, reflection back into the fiber is diminished and maximal coupling into the guided mode is achieved. In addition, this grating coupler shows strong fiber misalignment tolerance. Genetic algorithm (GA) is used to simultaneously optimize the grating and silicon nitride layer. The optimal design obtained from GA shows that the total in-plane optical coupling in C-band is enhanced from about 56.3% to 67%; meanwhile the back-reflection is reduced from 17.6% to 5.3%. What’s more, the device proposed here shows a wide-band character with a 1-dB-bandwidth of 54 nm. Such a design can provide an efficient and cost-effective solution for optical vertical coupling of a WDM application and low-cost fiber packaging for silicon PIC.