Abstract: We experimentally investigate the vibrational state distribution of the cations after carbon monoxide (CO) molecules are irradiated by strong laser fields. Vibrational-resolved fluorescent spectrum is observed, which can be well assigned as the A²Π(v′=0–3)→X²Σ⁺(v″=0–3) transitions of the cations CO⁺, indicating the strong field ionization of the inner molecular orbital of CO. Relative distribution of vibrational states A²Π(v′=0–3) of CO⁺ is retrieved from the measured spectrum and the Franck-Condon factors. It is found that the vibrational state distribution in strong field ionization of CO apparently deviates from the Franck-Condon-like distribution, but is in good agreement with the calculations, in which we include both the ionization rate and the overlapping between the vibrational wavefunctions of the neutral and cationic electronic states. The distribution of CO⁺(A²Π, v′) strongly depends on the laser intensity but is less dependent on the laser ellipticity. Analysis indicates the inter-nuclear distance-dependent ionization plays a significant role in the vibrational state distribution induced by strong field ionization of CO molecules.