Understand the structure of the ILs/air interface on a molecular level is significant. In this work, the liquid-air interface of a class of ILs containing 1-alkyl-3-methylimidazolium cations ([C(n)mim](+), n = 4, 6 and 8) was investigated by a sum-frequency generation vibrational spectroscopy experiment. The length of the alkyl chain of cations and anion type (tetrafluoroborate ([BF4](-)), thiocyanate ([SCN](-)), dicyanamide ([DCA](-)), hexafluorophosphate ([PF6](-))) were varied to determine their intrinsic effects to the micro-structure on the interface. Meanwhile, combining with the experiment, the orientation angle and surface density for cations and anions of ILs at the air interface was analyzed by molecular dynamics simulation. It is found that the butyl side chain of cations protrudes out into the air as well as the orientation angle between the side chain and interface is 30 degrees-50 degrees. Meanwhile, the anion volume of ILs has almost no influence on the butyl side chain and the methyl on the side chains tend almost planar to the interface with the increased chain length. This research is highly significant for the theoretical and quantitative understanding of the interface chemistry of imidazole-based ILs.