Lactobacillus plantarum was used as starter to ferment compound soybean milk. The changes of proteins, peptides, lipids, lactic acids, amino acids and 4 kinds of isoflavone were determined before and after fermentation. Three antioxidant models were used to analyze the changes of antioxidant activity of the compound soybean milk. The results showed that after fermentation the contents of peptides, lactic acids and amino acids in compound soymilk increased significantly; isoflavone glucoside declined while the content of isoflavone aglycone increased. The antioxidant activity of compound soymilk after fermentation was enhanced. The results of gas chromatography-mass spectrometry showed that alcohols, acids and esters aroma components increased significantly after fermentation, which improved the flavour of compound soymilk.