Main components of four kinds of commercial black garlic were determined and analyzed. Using the DPPH method, hydroxyl radicals and iron reduction method, the antioxidant activity of water-soluble extracts changes with the pH value were analyzed. The result showed that there were big difference in main components of different commercial black garlic. The highest value of moisture, protein, sugar, acid, total phenols, flavonoids and selenium content were 1.4, 2.2, 1.7, 1.9 and 1.7, 1.7, 4.1 times higher than the minimum value. Sugar-acid ratio ranged between 8 and 12. Antioxidant activity of black garlic extracts increased as pH values decreased, which decreased slowly as pH increased when pH was below 7, and it decreased quickly when pH was over 7. So, black garlic and its processing should be kept in an acid conditions to fulfil its antioxidant role.