The protective effect of methylnaphthoquinone MK-4 on the oxidation of carotenoids (β-carotene and lycopene) was studied. By simulating the ferricinduced oxidation of carotenoids at low pH in gastric juice,the effect of MK-4 on the retention rate of dietary carotenoids was evaluated.Taking β-carotene as an example,the type and quantity of reaction products of β-carotene in two different systems (without MK-4,adding MK-4) were identified by liquid chromatographymass spectrometry (LC-MS). The results showed that:MK4 had the best protection effect and the retention rate could be increased by 2 times when the oxidation of carotenoids was induced by Fe（Ⅱ）. For the gastric juice without MK-4 in the presence of Fe（Ⅱ）,there were the most types and quantities of reaction products of β-carotene,including cis isomers,epoxy compounds,aldehydes and degradation products,a total of 10 compounds. After the addition of MK-4,the reaction products of β-carotene were reduced to 7 compounds without aldehydes and degradation products. By comparing the HPLC of MK-4 in the micelles before and after the reaction,it was found that the protective effect of MK-4 on β-carotene did not cosume itself.