The method of Onsager and DFT(density functional theory) is used to optimize and calculate the geometry structure of o-xylene molecular and infrared frequency in the level of 6-31G* basis set.Meanwhile
the calculated o-xylene’s infrared frequency with its infrared frequency on the Sadtler standard was compared
the molecular structure of o-xylene was clarified and the infrared spectrum was classified.A conclusion is drawn that the Onsager method is fit for the research on the liquid o-xylene’s molecular structure and infrared spectrum after the applicability of the Onsager method is discussed.