NOVELTY - Common Fusarium, with preservation number of CBS131819 is new.    USE - For screening Fusarium for preparing viscosity reducing enzyme's; for fermentation of chunk's sweet potato stem useful in sweet potato fuel ethanol fermentation's application (all claimed).    ADVANTAGE - The common Fusarium is used for preparing viscosity reducing enzyme with high activity, less dosage, short time and good viscosity-reducing effect.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for following: 1) screening (M1) of Fusarium, involving preparing bacteria suspension using sweet potato chunk's stem rot, diluting by gradient to the screening culture medium, cultivating at 30 degrees C for 3 days, separating and culturing the PDA culture medium, cultivating at 30 degrees C for 7 days, congo red dyeing liquid dyeing, selecting bigger yellow transparent ring and Fusarium's strain, where the screening culture medium is 10 g/l xylan, 4.5 g/l yeast extract, 4.5 g/l (NH4)2SO4, 5 g/l NaCl, 2 g/l K2HPO4, 4 g/l MgSO47H2O, 15 g/l agar 15 with pH 6; 2) preparation (M2) of viscosity reducing enzyme involving: inoculating common Fusarium CBS131819 in the fermentation culture medium at 30 degrees C for 130 revolution per minute (rpm) for 4-5 days to obtain rough enzyme solution viscosity, where fermentation culture medium comprises is of following two kinds: one is fermentation culture medium comprising 20 g/l dry corn cob, 3 g/l peptone, 0.5 g/l yeast extract, 2 g/l KH2PO4, 5 g/l (NH4)2SO4, 0.15 g/l MgSO47H2O, 0.3 g/l CaCl2, 0.0005 g/l FeSO47H2O, 0.0014 g/l ZnSO47H2O, 0.0016 g/l MnSO4H2O, 0.0016 g/l COCl26H2O, 53.3 g/l Tween-80 (RTM: polysorbate 80); and other fermentation culture medium comprising 20 g/l dry corn cob, 2 g/l KH2PO4, 4.5 g/l NH4NO3, 0.15 g/l MgSO47H2O, 0.3 g/l CaCl2, 0.0005 g/l FeSO47H2O, 0.0014 g/l ZnSO47H2O, 0.0016 g/l MnSO4H2O, 0.0016 g/l COCl26H2O, 3.3 g/l Tween-80 with pH 6.5; 3) preparing (M3) viscosity reducing enzyme composition by using the viscosity reducing enzyme which is prepared by the method (M2); and 4) fermentation of chunk's sweet potato stem using viscosity reducing enzyme composition involving: reacting sweet potato stem chunk pulp with reaction enzyme solution, at 50 degrees C for 150 rpm for 2 hours, where reaction enzyme solution is viscosity reducing enzyme composition.