The space mutagenesis Lactobacillus paracasei A-4-2 with excellent acid production capability, which was preserved in the laboratory, was selected as the object. The characteristics of A-4-2, including morphology, tolerance, extracellular polysaccharide content and gene expression of βgalactosidase were measured by electron microscopy, low pH value and fluorescence quantitative PCR. The differences of the acid resistance, bile salt, extracellular polysaccharide, surface hydrophobicity and gene expression of βgalactosidase were also evaluated between wild strains and mutant ones. The results showed that there was no significant difference of the morphology between the mutant strain and the wild ones. The survival rate of mutant strain was significantly higher than that of wild one (P＜0.05), which was cultivated for 1h or 2h at pH value of 3.5. For the acid capacity evaluation of fermentation strains after 7d cultivation at different pH values, mutant strain on the number of living bacterium was significantly higher than that of wild one (P＜0.05), however, the ability of acid resistance, bile salt and extracellular polysaccharide was not changed, the surface hydrophobicity showed an significant decrease. The activities of βgalactosidase and gene expression were significantly higher than those of the original strain (P＜0.05). The results indicated that the activity of βgalactosidase was closely related to the acid production capability.