The electronic structure and optical properties of Sn-doped ZnO for different doping concentrations are calculated by using the first-principles under the framework of density functional theory with the generalized gradient approximation and the Perdew-Burke-Ernzerdorf functions. The effect of doping concentration on the crystal structure, band structure, density of state, and optical properties is studied. Meanwhile, according to the calculated band structure and charge density of difference, the effect of doping site on the calculated results is investigated. The results show that with the increasing Sn doping concentration, the ratio of lattice constants c to a is stable, and the doped structure does not distort. The total energy of the doped system increases gradually, thus the stability weakens, and the band gap decreases first and then increases. The doped SZO (Sn∶ZnO) system becomes an indirect band gap semiconductor, and a large number of conductive carriers, which are contributed by the doped Sn atoms, are introduced to the bottom of the conduction band. As a result, the conductivity is significantly improved. Moreover, a V-shaped curve occurs between the Fermi level and the top of the valence band, which shows the characters of half-fill state. After doping, density of state peak of the valence band moves to the lower energy by about 1.5 eV. The donated and received electrons in the same-layer doping are more than those in the neighbor-layer doping and the alternate-layer doping whereas the former band gap is smaller than the latter. The absorption edge has a red shift, and the ultraviolet absorption capacity is enhanced significantly. The imaginary part of the dielectric function increases, and the primary transition peaks shift to higher energy. The calculated results show that SZO (Sn∶ZnO) is a good transparent conductive film.