The invention firstly provides a method of producing ethanol by biomass fermentation. The method includes: (1) a step of pretreatment, namely a step of smashing raw materials, conveying the raw materials, mixing with an acid solution, cooking and vacuumizing; wherein the raw materials are contacted with an acid or an anhydride in a spraying manner; (2) a step of enzymatic hydrolysis; (3) a step of fermentation, namely a step of culturing C5/C6 co-fermentation microorganisms by adoption of a seed culture medium for 12-24 h, inoculating, performing amplification culture, transferring the microorganisms after the amplification culture into an enzymatic hydrolysate, and fermenting for 24-72 h; and (4) separating the fermentation broth to obtain the ethanol.