This study aimed to improve the acid tolerance of Lactobacillus casei Zhang and compare the stress response of the parental strain and the acid-resistant mutant during acidic conditions. Adaptive evolution was conducted for 70 days to generate acid-tolerant L. casei. The evolved mutant lb-2 exhibited more than a 60% increase in biomass as well as a 13.6 and 65.6% increase in concentrations of lactate and acetate, respectively, when cultured at pH 4.3 for 64 h. Lactic acid tolerances of the parental strain and the evolved mutant were determined. As a result, the evolved mutant showed a 318-fold higher survival rate than that of the parental strain. Physiological analysis showed that the evolved mutant exhibited higher intracellular pH ($pH_i$), $NH_4{^+}$ concentration and lower inner membrane permeability than that of the parental strain during acid stress. Moreover, higher amounts of intracellular arginine and aspartate were also detected in lb-2 under acid stress. Validation of the relationship between the acid tolerance and the intracellular arginine and aspartate accumulation was conducted by experiments that showed the survival of L. casei at pH 3.3 was improved 1.36-, 2.10-, or 3.42-fold by the addition of 50 mM aspartate, arginine or both of them, respectively. Taken together, results presented here not only supply an effective way to select acid-resistant strains for the food industry, but also contribute to reveal the mechanisms of acid tolerance and provide new strategies to enhance the industrial utility and health-promoting properties of this species.