Equation of state and phase transformation under high pressure of ZnS in zinc blende (ZB) and rock salt (RS) structures have been calculated by means of plane wave pseudo-potential method (PWP) with generalized gradient approximation (GGA). The electronic density of states, band structure and optical properties of change mechanism have been discussed near the point of phase transformation. The results reveal that the transition pressure of ZnS from ZB to RS phase is 18.1 GPa by the equation of state, but it is 18.0 GPa obtained with the enthalpy equal principle. The sp3 hybrid orbital has not been eliminated and the metalic behavior of RS ZnS is enhanced significantly in the structural phase transition process. By comparing RS phase with ZB one, the main peak of the dielectric constant in RS ZnS becomes higher and shifts to the lower energy direction apparently, and other dielectric peaks are also extended to lower energy direction, at the same time the electronic transitions are enhanced in the low energy region.