Electronic structure and optical properties are systematically investigated by using first-principles plane-wave calculation method based on density functional theory, and the changes in the band structure, density of states and optical properties are comparatively analysed for GaN under pressure. The calculated results indicate that Ga—N bond lengths become shorter, the populations become larger, the covalent becomes stronger and the ionic becomes weaker when the pressure increases. The calculated electronic structure shows that the conduction bands move to high energy direction, while the whole valence bands shift to low energy direction, the band gap becomes wider, the hybridization of 3d states electron of Ga atoms and 2p states electron of N atoms is enhanced significantly. Results of optical properties reveal that absorption spectrum occurs absorption edge near 1.6 eV when there is no stress. With the pressure increasing, the complex dielectric function and absorption spectra of GaN move towards high energy, the absorption spectrum occurs obvious blue shift and the photoelectric conversion efficiency is improved.