Abstract: The influence of osmotic pressure on the production of L-lactic acid by Lactobacillus paracasei was studied. First, NaCl was used as an osmotic regulator to study the influence of osmotic pressure on the growth and acid production of the Lactobacillus paracasei. By comparing different concentrations of NaCl (0²mol/L) , 1mol/L NaCl was chosed as an osmotic pressure filter condition. Then the role of three different osmoprotectants in the production of L-lactic acid was studied in this filter condition. Results indicated that 2g/L proline offered optimal osmoprotection. Finally, in tests of 5L batch fermentation, 2g/L proline was added to the broth after 24h of fementation, and the productivity of L-lactic acid and consumption rate of glucose in phase II were observed to be increased by 50.34% and 39.61%, respectively, as compared with the control. Moreover, the analysis of the main extracellular byproducts demonstrated that proline addition could decrease the formation of byproducts efficiently and increase the production of L-actic acid. This method of enhancing L-lactic acid production by the addition of an osmoprotectant might potentially provide an alternative approach for enhanced production of other organic acids.