The invention discloses a method for preparing lysine through fermenting. The method comprises the steps: inoculating a lysine fermentation strain to a lysine fermentation culture medium under the conditions of producing lysine, and carrying out fermentation culture under the conditions of feeding a carbon source and feeding a nitrogen source. The method is characterized in that the content of SO4<2-> in an initial culture medium is 0.8-4.8g/l, the fed nitrogen source consists of ammonium sulfate and ammonium chloride, and the amount of fed ammonium sulfate meets the requirements on that the content of SO4<2-> in fermentation liquor is 1-5g/l when the OD (Optical Density) value of the fermentation liquor during fermentation is 0.01-0.3, the content of SO4<2-> in the fermentation liquor is 3-10g/l when the OD value of the fermentation liquor is 0.3-0.85, and the content of SO4<2-> in the fermentation liquor is 1-8g/l when the OD value of the fermentation liquor is greater than 0.85. The method disclosed by the invention has the advantages that an extracting process route can be shortened, the consumption of raw and auxiliary materials is lowered, the extracting cost is reduced, and the production index of lysine is not affected.