Biochar often refers to a carbon-rich product obtained by pyrolyzing biomass at relatively low temperatures (<700 ℃). The increasing interest in the applications of biochar in soil amendment and environmental contaminant reduction has spurred many studies in recent years. Due to the high content of ash in biochar, the adsorption capacity of bichar is limited. The ash of biochar formed during pyrolysis has great influence on the structure characteristics and spectral characteristics of biochar. The ash of biochar is expected to enchance their efficacy and minimize their influence. For this purpose, the corn straw biochar made at two different temperatures (400 and 600 ℃) was used to study the effect on the structure characteristics of biochars under the acid elution. The samples were characterized with element analysis, Brunaner-Emmett-Teller-N2 surface areas, pore size distributions and micro infrared for structure analysis in this study. The result indicate that: the elution solvent could remove the by-products and improve the adsorptive capacity of biochars. (1)After acid elution, the carbon content, hydrophobicity and the aromatic functional group content were increased; the polarity of biochar was decreased. (2) After acid treated, the specific surface area of the biochars was significantly increased, the increment of two biochars was 3.46 times and 6.75 times; the pore volume and the mesoporous quantity were also enhanced. (3) After acid elution, the infrared analysis is similar, but the content of key functional groups was different significantly. The result of infrared analysis showed that, several absorbance peaks were decrease at 3 398~3 447, 2 924~3 056, 1 378~1 439 cm-1, and the acid treated can make the content of aliphatic chain char and hydroxyl reduce. (4) Biochar with low polarity and high aromaticity displayed a superior 2,4- dichlorophenoxyacetic acid adsorption capacity. Acid elution made the surface of biochar exposed more organic surface and porosity and it is one of the most important factors for the adsorption of biochar. The date with elemental analyzer, BET, FTIR techniques and batch experiment allowed us to develop a better understanding of evolution of biochar properties during the acid elution and these results have significant implications for using biochar as a soil additive for soil pollution.