[Background] As an important microorganism in fermentation, lactic acid bacteria are confronted with severe acid stress during the process of production. [Objective] We analyzed the specific mechanism how aspartic acid improved acid stress resistance of L. lactis NZ9000. [Methods] The effects of aspartate on transcription levels of key genes in energy production and amino acid metabolic pathways were analyzed by fluorescence quantitative PCR under acid stress. The content of aspartic acid was increased by overexpression of L-asparaginase. [Results] The main mechanism of aspartic acid to improve the acid stress resistance was generating oxaloacetate and glutamate in the role of transaminase. Oxaloacetate entered three carboxylic acid cycle and can provide more energy for cells; glutamate promoted acid resistance through glutamic acid decarboxylase pathway. The transcription levels of genes of glycolysis and three carboxylic acid cycle pathway were up-regulated and the content of intracellular ATP was 42 times as high as that in the control group at pH 4.0 for some time. The intracellular glutamate content was 1.99 times higher than that of the control. The survival rate of recombinant strain obtained by overexpression L-asparaginase was about 11.11 times as high as that of the control at pH 3.6 for 0.5 h. [Conclusion] Aspartic acid could improve acid stress resistance of L. lactis NZ9000.