In this paper, we report the first study on interaction of ethanol with molocular oxygen pre-adsorbed on a polyorystalline silver surface by optical second harmonic generation (SHG) in an ultrahigh vacuum system. It is found that the interaction consists of two process, one is the enhanced adsorption of ethanol induced by molecular oxygen: the other is the reaction of ethanol with molecular oxygen. From the temperature spectrum measured by heating the adsorbed system, we obtain the desorption temperature of reactant and interactive strength of ethanol with molecular oxygen.