  In present paper we develop an analytic theory for the harmonic generation of
symmetric diatomic molecular ions beyond two-level model, emphasizing the
influence of charge-resonance (CR) states those are strongly coupled to
electromagnetic fields for large internuclear distance. With taking into
account the continuum states that is ignored in the two-level model and become
important for intense laser case, our model is capable to produce spectrum for
the whole range of harmonic orders consisting of a molecular plateau due to the
CR transition and an atomic-like plateau for a long-wavelength excitation, and
in good agreement with numerical results from directly solution of the
Schr\"odinger equation. Our theory also identifies the crucial role of the CR
states in the fine structure of harmonic spectrum and shows that the harmonic
generation in molecular system can be effectively controlled by adjusting the
internuclear distance.
