Extractions of flavonoid compounds in onion with ethanol, using the optimization factors of single factor and orthogonal design experiments, the flavonoids extraction rate of onion peel was 21.58%. The effect of quercetin on the differentiation of 3T3-L1 preadipocytes was best by separating the flavonoids from the liquid phase and acting on the 3T3-L1 preadipocytes. The expression of fat differentiation related gene FASmRNA was measured in 3T3-L1 adipocytes induced by flavonoid monomers of onion skin for eighth days. The results showed that the relative expression of FAS in 3T3-L1 preadipocytes induced by mRNA in quercetin group was significantly lower than that in control group after 8D.