The electronic structure and optical properties of oxygen vacancy and La-doped Cd₂SnO₄ were calculated using the plane-wave-based pseudopotential method based on the density functional theory (DFT) within the generalized gradient approximation (GGA). The formation energy of different oxygen vacancies showed that the V_O2 oxygen vacancy was easy to obtain in experiments. The Bader charge analysis is implemented to directly observe the electron transfer and distribution for each atom. The calculated band structures show that when the oxygen vacancy was introduced, the impurity energy level appeared in the band gap. The impurity levels induced by oxygen vacancies were mainly composed of O 2p orbits and a very small amount of Cd 4s orbits. After La doping based on the V_O2 oxygen vacancy of Cd₂SnO₄, the Fermi energy level entered the conduction band and overlapped with the conduction band which increased the conductivity, and the band gap value increased to above 3.0 eV. The optical calculation results showed that the transmittance of the V_O2 oxygen vacancy of Cd₂SnO₄ increased in short wavelength (<600 nm), the reflectivity increased in the infrared region compared with Cd₂SnO₄, and the transmittance increased to 90% in visible light region after La doping.