Abstract: Torrefaction and acid-washing are both the pretreatment methods to improve the quality of biomass. Torrefaction can remove oxygen in the biomass, and acid-washing can effectively remove alkali metals and alkaline earth metals (AAEMs). Both oxygen and alkali metals and alkaline earth metals have effect on the quality and yield of pyrolysis oil. However, after torrefaction, there are still a lot of alkali metals and alkaline earth metals remaining in the biomass. Therefore, it is necessary to take appropriate pretreatment methods to remove alkali metals and alkaline earth metals. But acid-washing can not remove oxygen from the biomass. Therefore, for improving the quality of biomass, torrefaction and acid-washing have some limitations. At present, there are many researches on torrefaction and acid-washing, but the the coupling effect of the torrefaction and acid-washing on pyrolysis characteristics is rarely reported. In this paper, a method of two-stage pretreatment by acid-washing and torrefaction was proposed and the coupling effect of the torrefaction and acid-washing on pyrolysis characteristics of corn stalk was studied. The acid liquor was taken from the aqueous portion of the pyrolysis oil from a demonstration project. If the aqueous portion can be used for washing biomass, it is beneficial to increase the benefit of the system and realize the internal circulation of industrial materials. The torrefaction temperature is 230, 260 and 290 ℃. It is found that the AAEMs can be effectively removed by acid-washing, and the removal rates of K, Na, and Mg are 97.53%, 81.38% and 84.86%, respectively. Two-stage pretreatment can significantly reduce O/C ratio; compared to the original corn stalk, the O/C of semi-coke with washing-torrefaction under 290 ℃ reduces by 25.32%. Two-stage pretreatment can obviously reduce the unfavorable effects of torrefaction on oil yield; compared to 290 ℃ torrefaction semi-coke, the oil yield of semi-coke with washing ?290 ℃ torrefaction increases by 127.66%. Compared to original corn stalk, there is still a 3.88% increase in the oil yield. Two-stage pretreatment can significantly enhance the carbohydrate content in pyrolysis oil, while reducing the content of phenols and acids. There is no carbohydrate in the pyrolysis oil of corn stalks and torrefaction semi-coke. With the increasing of torrefaction temperature, the relative content of carbohydrate in the pyrolysis oil of washing-torrefaction semi-coke gradually increases. For semi-coke with washing ?290 ℃ torrefaction, the content of the carbohydrate in the pyrolysis oil is 45.89%. The relative content of phenols in the pyrolysis oil of corn stalks is the highest, reaching 25.92%. With the increasing of torrefaction temperature, the content of phenols in the pyrolysis oil decreases gradually from 18.42% of sampling with acid-washing to 9.76% of sampling with acid-washing and 290℃ torrefaction. For straw without pretreament, the relative content of the acids in the pyrolysis oil is 20.79%. With the increasing of torrefaction temperature, the relative content of acids in the pyrolysis oil of the torrefaction semi-coke and the washing-torrefaction semi-coke decreases. For 290 ℃ torrefaction semi-coke and the washing and 290℃ torrefaction semi-coke, the content falls to 16.81% and 6.31%, respectively. Contents of other chemical components such as ketones and furans reduce to a certain extent, but there is a small increase in aldehydes content. On the whole, the two-stage pretreatment not only improves the yield and quality of the pyrolysis oil, but also finds a good way for the use of the aqueous portion.