Solar UV radiation induces the dissociation of the chlorinated alkyl bromide with the product of free bromide radical and chlorine radical, which seriously damage the ozone layers. The molecular structure optimization and infrared spectra of chlorinated alkyl bromide CnH2nBrCl(n=1～16)were calculated by the[B3LYP/6-311++G(d, p)] basis set levels using density functional methods (DFT) approaches. Via comparison and discussion, the tendency of molecular structural parameters, like bond length, bond angle of CnH2nBrCl changing with the alkyl chain length increasing (n=1～16) are calculated. Meanwhile, the changing rules of C-Br and C-Cl vibration with the alkyl chain length increasing are also determined. Dependence of the molecular configuration and spectra of CnH2nBrCl on alkyl chain length (n) are obtained.