It is needed to improve the tight-binding SSH model which is used to describe the conducting polymers, since the conducting polymers are the systems with wide energy band. In order to study one-dimensional lattice instability of conducting polymers more precisely, it is necessary to go beyond the tight-binding approximation. In this paper the Wannier function method used in stable lattice is extended to calculate the electron energy band of insta-ble lattice. The numerical results show that our method has good convergence, the accuracy can be better than 1% when the third neighbor is included. Meanwhile the dimerization caused by the lattice instability can also be determined precisely.