Crystal structure and electronic and optical properties of LaBr3 under high pressure are systematically investigated by first-principles method within the local-density approximation (LDA). The calculated lattice parameters and bulk modulus are in good agreement with the experimental data reported. The results of the band structure calculation show that LaBr3 is a direct-band-gap insulator without pressure, and the top of the valence band is mainly composed of Br 4p states while the lower part of the conduction band is dominated by the La 5d states. Under high pressure, LaBr3 gradually transforms into an indirect-band-gap insulator while the band gap presents a basically linear reduction trend with the increase of pressure. Analysis of the optical properties shows that the transmittance of LaBr3 reaches 80% in the visible and infrared region, indicating that LaBr3 is an ideal transparent material. With the increase of pressure, two higher peaks of the imaginary part of the dielectric function, the optical absorption edge and the optical transmittance edge are red-shifted. Furthermore, the static dielectric constant, the refractive index and the reflectivity of LaBr3 increase gradually. In a word, the involved results show that the high pressure effectively modulates the electronic structure and optical properties of LaBr3, which can provide a theoretical basis for the design and application of LaBr3 optoelectronic materials.