Abstract: Starting from the electron continuity equation and considering the effect of a weak geomagnetic shear, after a direct two point regularization, a equation for the evolution of the correlation function of the electron density fluctuation <δn(1)δn(2)> was established and analytically resolved. The resulting spectra are in fair agreement with the observational turbulence spectra of the large scale equatorial spread-F (λ≥100m) in spectral shape, intensity and indexes by the method of least variance fitting. Thus, a unified explaination of the experimental results generally for both the medium and intermediate scale are given, and the similarity of the shapes for above and below the Fpeak, or for parallel and perpendicular to the earth's surface is also explained.