Combining molecular dynamics (MD) and quantum chemistry (QM) simulation, the association mechanisms of acetic acid (AA) systems were examined. DFT methods were proposed to compare the hydrogen-bonding energies of variable acetic acid dimers and definitely provided the stable dimers configuration. Geometry parameters of dimers were also obtained by QM calculations, which were taken as the characteristic criteria for further MD analysis. Proportion of different acetic acid dimers in gas phase was obtained by Radial distribution function (RDF) analysis, and cyclic dimer with two O1Ho hydrogen bonds was demonstrated as the most stable structure. While in the more complex liquid phase, the linear chain form was proved to be the most stable one. Furthermore, in the acetic acidcrotonaldehyde solution, the association configuration of acetic acid changed from the linear chain form to the cyclic dimer structure as the acetic acid concentration decreased gradually. This result would be significant for the chemical separation process of acetic acidcrotonaldehyde solutions.