Abstract: In order to maximize the liquid yield and glucide content in biomass pyrolysis products, acid wash pretreatment was usually adopted. The analysis of microstructure and polymerization degree shows that the acid wash can alter the cellulose morphology and decrease the polymerization degree largely. A series of experiments were carried out to study the effect of acid wash on the cellulose rapid pyrolysis. Experimental results indicate that under the acid pretreatment the yield of bio-oil decreases, while the yield of gas and char increases. The effect would be greater with the increase of acid concentration. Sulphuric acid has a stronger restraint on the formation of bio-oil than hydrochloric acid and phosphoric acid. It is suggested that the acid has a catalysis to the cross linking and dehydration reaction of cellulose. According to the GC-MS analysis of bio-oil, the existence of acid would not alter the bio-oil components, but change the relative contents of bio-oil compounds. High-concentration acid wash can restrain the formation of levoglucosan by catalyzing the dehydration and cross linking reaction.