86％。 By using single-factor test and orthogonal test, the optimized seed medium and feeding medium were obtained, with the aim to study the stability of the production of lactic acid by Rhizopus oryzae though semi-continuous fermentation. The optimization culture medium was as follows: glucose 120g/L，(NH4)2SO4 2g/L，NaH2PO4 0.16g/L，KH2PO4 0.14g/L，MgSO4?7H2O 0.2g/L，ZnSO4?7H2O 0.22g/L，CaCO3 60g/L; the feeding medium was as follows: glucose 100g/L，（NH4）2SO4 2g/L，KH2PO4 0.1g/L，ZnSO4?7H2O 0.33g/L，MgSO4?7H2O 0.15g/L，CaCO3 50g/L. In flask culture 30 repeated stable fermentation were carried out, and 20 batches were carried out in a 5 L stirred tank fermentor. The highest lactic acid yield from glucose was up to 86%.