Potato starch residue was used as raw material , a single factor test was used to determine the lactobacillus casei L-lactic fermentation in the amounts of CaCO3 addition, fermentation temperature, residue saccharification of starch concentration, the optimal dosage range of fermentation time. With 4 factors and 3 levels of 4 orthogonal test of L-lactic acid by fermentation. The order was: the fermentation temperature> saccharification concentration> fermentation period >CaCO3 dosage. Optimization was as follow : residue saccharification of starch concentration was 80g/L, fermentation temperature was 37°C, CaCO3 addition level was 60g/L, fermentation time was of 60h. Fermentation conditions for this verification test, L-lactic acid content was 72.3g/L, compared with 15.1% before optimization.