The conversion of lignocellulose into a value-added chemical with high selectivity is of great significance but is a big challenge due to the structural diversities of biomass components. Here, we have reported an efficient approach for the one-step conversion of raw lignocellulose into diethyl maleate by the polyoxometalate ionic liquid [BSmim]CuPW₁₂O₄₀ in ethanol under mild conditions. The results reveal that all of the fractions in biomass, i.e., cellulose, lignin and hemicellulose, were simultaneously converted into diethyl maleate (DEM), achieving a 329.6 mg g⁻¹ yield and 70.3% selectivity from corn stalk. Importantly, the performance of the ionic liquid catalyst [BSmim]CuPW₁₂O₄₀ was nearly twice that of CuHPW₁₂O₄₀, which can be attributed to the lower incorporation of the Cu²⁺ site in [BSmim]CuPW₁₂O₄₀. Hence, this process opens a promising route for producing bio-based bulk chemicals from raw lignocellulose without any pretreatment.