NOVELTY - Fermentation of corn straw comprises preparing liquid culture medium; sterilizing and inoculating with Rhizopus oryzae mutant-2; fermenting while stirring with aeration amount of 0.35-0.45 L/Lx minute; and crushing to get 40 mesh sieve corn straw powder, adding 1-2 times water, wet milling for 1-2 hours in ball mill, transferring to hydrolysis reactor, adding 5-6 times water, treating with sulfuric acid solution to adjust pH to 2-3 and hydrolyzing for 1-1.5 hours to 110-130 degrees C; adjusting pH value to 4.5-5.5 using aqueous ammonia, solid-liquid separating and concentrating.    USE - Method for fermenting corn straw used for preparing L-malic acid (claimed).    ADVANTAGE - The method yields L-malic acid up to 3%, increase L-malic acid production by 51.76% and decreases impurities such as fumaric acid and succinic acid content by 90.12% and 81.34%, respectively.    DETAILED DESCRIPTION - Fermentation of corn straw comprises taking 85-115 g/L corn straw hydrolyzate as culture medium substrate, adding 3.5-4.5 g/L ammonium sulfate, 0.25-0.35 g/L magnesium sulfate, 0.020-0.030 g/L ferrous sulfate, 0.45-0.55 g/L potassium dihydrogen phosphate, 0.05-0.15 g/L zinc sulfate and 70-90 g/L calcium carbonate to get liquid culture medium; sterilizing and inoculating with 106-107 colony forming unit (CFU)/minute Rhizopus oryzae mutant-2; fermenting at 31-33 degrees C in fermentation tank while stirring for 70-75 hours with aeration amount of 0.35-0.45 L/Lx minute; and crushing to get 40 mesh sieve corn straw powder, adding 1-2 times water, wet milling for 1-2 hours in ball mill, transferring to hydrolysis reactor, adding 5-6 times water, treating with sulfuric acid solution to adjust pH to 2-3 and hydrolyzing for 1-1.5 hours to 110-130 degrees C; adjusting pH value to 4.5-5.5 using aqueous ammonia, solid-liquid separating and concentrating to 85-115 g/L. The Rhizopus oryzae mutant-2 is breed by taking 10-30 Gy rice Rhizopus oryzae mutant-2 isotope 60Co as starting strain (Rhizopus oryzae CICC41160 mutagenesis) and irradiating for 20 minutes; and screening with 6 ml/L propylene alcohol in flat panel to get mutant strain-1, inoculating with 60Co and screening with 7 g/L fluorine acid in flat panel to get mutant strain-2.