The invention discloses a method for producing docosahexaenoic acid by fermentation tank substrate feeding, and relates to docosahexaenoic acid. The method comprises the following steps: transferring a strain into a plate medium for cultivation to obtain an activated strain, wherein the strain is Schizochytrium sp. LU310, with the preservation number of CGMCC No. 12528; inoculating the obtained activated strain in a plate bacterial colony into a conical flask with a seed culture medium for cultivation; inoculating an obtained fermented culture solution into a fermentation culture medium for cultivation, automatically controlling the pH of the fermented solution by feeding 2M of NaOH or 1M of citric acid, sampling every 4 h to measure the glucose concentration, and sampling every 12 h to measure the biomass, the yield of grease and the yield of DHA.