Resonance enhanced two-photon ionization (RE2PI) spectra of KRb molecule were obtained in a very cold supersonic molecular beam over the range of 460~505 nm ($19800~21700 cm^{-1}$). Three progressions were newly observed in this region. The absolute vibrational quantum numbers and the molecular constants of this molecule were determined by isotopic shift of the vibrational energy for two progressions. Experimental results were compared with an ab initio calculation of excited electronic states. As results, the possible excited electronic state observed near 500 nm is expected 4 $^3\Sigma^+$ and it seems that the progression near 490 nm corresponds to the 5 $^1\Sigma^+ - X $^1\Sigma^+$ transition. And the excited electronic state observed near 470 nm is expected 6 $^1\Sigma^+$. Ion pair character of the outer well of 5 $^1\Sigma^+$ and $6 ^1\Sigma^+$ states in potential curves is reflected in an anomalous increase in $ΔG_v$ versus v. The electronic term values and vibrational frequencies of $^39K$ $^85Rb$ are determined as $T_e=19861.4±0.04 cm^{-1}$ and $ω_e=63.50±0.08cm^{-1}$ for the 4 $^3\Sigma^+$ and $T_e=20382.0±0.6cm^{-1}$ and $ω_e=80.34±0.92cm^{-1}$ for the 6 $^1\Sigma^+$ state. Potential well depth($D_e$) of the 6 $^1\Sigma^+$ state was calculated from the measured electronic term value and asymptote. $D_e= 0.575±0.02 eV$ for the 6 $^1\Sigma^+$ state.