CN radical plays an important role in the combustion of carbides.The low-excited state of CN was investigated by the ab initio calculation.Firstly,the molecular orbitals were determined by the Hartree-Fock self-consistent field approximation and optimized to use the complete active space self-consistent field theory.Then,the potential energy curves of three low excited states (X2Σ+,A2Π,and B2Σ+) for CN in the range of nuclear distance from 006 to 07 nm were obtained through multi-reference configuration interaction method.Considering the relativistic effects,the aug-cc-pV5Z-dk correlation-consistent basis set was selected in order to further improve the accuracy of the calculation.The vibration-rotation energy and constants of these electronic states were derived via solving the Schrodinger equation.Through fitting date listed above,the spectral constants(Te,Re,ωe,ωeχe,Be,αe),which are basically consistent with the experimental results,and Franck-Condon factors of three low excited states were acquired.Simultaneously,the transition dipole moments of the A2Π-X2Σ+ electric transition of CN have been computed,deducing the radiation lifetime τ and the oscillator strength f00 of the five low vibration levels.The calculated radiation lifetime is approximately 4 μs in accordance with the experimental measurements,which helps the study of the combustion dynamic.