Using the first-principles plane-wave pseudopotential method, based on the density function theory, the electron structure and optical properties of GaAs (100) \beta 2(2 \times 4) and GaAs (100) (4 \times 2) reconstructions are calculated. The formation energy of As-rich \beta 2(2 \times 4) reconstruction is minus and the formation energy of Ga-rich (4 \tiems 2) reconstruction is positive; As-rich \beta 2(2 \times 4) reconstruction is stable and Ga-rich (4 \times 2) reconstruction is unstable. Ga-rich (4 \times 2) reconstruction owns lower work function. The electrons at two reconstructions both move into the bulk and form a band-binding region. Both the absorption and the reflectivity of As-rich 2(2×4) reconstruction are smaller than the Ga-rich (4×2) reconstruction. Asrich \beta 2(2 \times 4) reconstruction is more benefit for the movement of photos through the surface to emit photoelectrons.