In order to explore the influence of structure and properties of Li-doped potassium tantalite niobate（ KTa_（1-x）Nb_xO_3

KTN）

the model of KTN（ KTa_（0.5）Nb_（0.5）O_3） and KL0.125TN（ K_（0.875）Li_（0.125）Ta_（0.5）Nb_（0.5）O_3） were established in this paper.The geometry optimization

electronic structure

optical properties and elastic properties of the model were calculated using first-principle density functional theory.The results show that the lattice constants were decreased from 3.991 to 3.986 and the band gap was increased after Li-doped.In the range of visible light

the refractive index decreased with increasing wavelength

which belongs to the normal dispersion and the Cauchy parameters A

B and C are 1.796 9、- 0.010 5、0.008 7

respectively.The extinction coefficient is very low in the range of visible light

especially when the wavelength is bigger than 450 nm

which indicates that the transparency was excellent.In addition

the independent elastic constants

bulk modulus

shear modulus and Young modulus of KL_（0.125） TN crystal

decrease comparing with the KTN crystal

whereas the Poisson ratio increases.The increasing flexibility and decreasing brittleness are propitious to the practical application.