The effect of pressures to ZnO structures and optical properties was studied. All the work was performed by adopting the first-principles based on the density function theory(DFT). The electronic structures were optimized under different pressures. The dielectric function, complex refractive index, absorption coefficient and reflectivity for ZnO were calculated and analysed at 0, 2, 4, …, 12 GPa. The results indicate that, with pressure increasing, the lattice constants and cell volume become smaller, the internal parameter u increases, bond of Zn-O becomes shorter, the covalency is enhanced and the band gap becomes wider. The spectral graphs show that pressures have little effect on optical properties in low energy region, but all the optical properties have obvious blue shift with the increase of pressures in high energy region.