We found that an out-of-plane vertical electric field of 1.0 V/Ang helps to maintain the thermodynamic and kinetic stability of monolayer CdI2.The results indicated that the electric field modulates monolayer CdI2 to produce the Mexican-hat electronic state and the giant Stark effect of the vertical electric field on monolayer CdI2 originates from electric field lifting its conduction band. The results based on HSE06 + SOC calculations show that electric field induces strong spin polarization, leading to significant energy level splitting and spin flipping in the valence band. Based on GW0 + BSE, the electric field broadens effective optical response range of monolayer CdI2, the new peak in the optical absorption spectrum under electric field indicates that electric field helps to diminish excitonic effect of monolayer CdI2.