The invention belongs to the field of fermentation engineering, relates to a method for producing DHA (Docosahexaenoic Acid) by utilizing microorganism fermentation and in particular relates to a method for producing mixed oil containing the DHA by utilizing industrial fermentation of schizochytrium strains. More specifically, the invention relates a method for cultivating microorganisms for producing the DHA, wherein the dissolved oxygen saturation is controlled to be 5 percent to 10 percent from 36h to 60h of a culture process of a fermentation tank, and/or a nitrogen source is not added any more from 36h to 60h of the culture process of the fermentation tank. When the method provided by the invention is used for preparing the DHA, the yield is high and the purity is high; large-scale industrial production of the DHA is facilitated.