  We study the mobility edges in a variety of one-dimensional tight binding
models with slowly varying quasi-periodic disorders. It is found that the
quasi-periodic disordered models can be approximated by an ensemble of periodic
models. The mobility edges can be determined by the overlaps of the energy
bands of these periodic models. We demonstrate that this method provides an
efficient way to find out the precise location of mobility edge in
qusi-periodic disordered models. Based on this approximate method, we also
propose an index to indicate the degree of localization of each eigenstate.
