To improve the neutral DES (choline chloride/glycerol) pretreatment performance, three environmentally friendly heteropoly acids (phosphotungstic, phosphomolybdic and silicotungstic acids) were used as catalysts. Pretreatment with silicotungstic acid at 120 °C for 3 h resulted in 97.3% of enzymatic digestibility at an enzyme loading of 15FPU/g substrate, which was approximately eight times more than that of raw samples. More importantly, 80% of glucose yield was obtained within 12 h. Simultaneously, 81.8% of ethanol yield was achieved in the SSSF process. The efficient conversion was ascribed to the significant delignification (89.5%), which resulted in the exposure of more accessible specific surface area. This was attributed to that the proton (H) from heteropoly acids could significantly contribute to the lignin degradation. Intriguingly, trace acetic acid (0.39 g/L) and HMF (0.21-0.95 g/L) in the pretreatment liquor were produced without any significant deleterious effects. These discoveries provide new insights for efficient biomass conversion under mild conditions.