Using first-principles calculations based on the density functional theory, we systematically study the geometry structure, electronic structure and optical properties of the small size (n, 0)-type TiO2 nanotubes (D2 unit decreases with the diameter increasing, and the nanotubes become more stable. At a diameter of about 14 Å, a configuration change occurs. Band structure analysis shows that electronic states of TiO2 nanotubes are localized, and the conductivity is better for nanotubes with small diameters (D2 nanotubes shift from direct band gap to indirect band gap. And the band gap increases with diameter increasing, because π orbital overlap effect is greater than the quantum confinement effect. Owing to the competition between the two effects, the peaks of the dielectric function ε2(ω) will become redshifted or blueshifted. When its diameter is larger than 9 Å ((8, 0) tube), the optical absorption of TiO2 nanotubes will be significantly enhanced.