The present disclosure relates to a hydrolysate of a mixture of lignocellulosic biomass and seaweed biomass. By mixing seaweed biomass with lignocellulosic biomass and then preparing a hydrolysate, lignocellulosic biomass-derived acetic acid is consumed together with seaweed biomass-derived mannitol. As a result, high sugar productivity can be maintained while reducing fermentation inhibitors. Because the present disclosure can solve the problem of lignocellulosic biomass of decreased fermentation efficiency due to lignocellulose-derived fermentation inhibitors and the problem of seaweed biomass of very low productivity in spite of long fermentation time, the hydrolysate according to the present disclosure may be used to produce biofuels and biochemicals economically.