Abstract: The chromatographic analysis of benzene homolog in formation water is described. The detection limit of this method is 0.01ul/1. The recovery is around 90%, and the relative error is less than 6%. The compositional character of the benzene homolog and its geochemical significance have also been studied. According to the regularity of the variations in composition, toluene/benzene is considered to be an effective parameter in petroleum geology and geochemical exploration.