To research the processing principle of Dangshen ［Codonopsis pilosula (Franch.) Nannf］.Methods:We used HPLC method to compare differences of chemical composition between crude Dangshen and its processed product, isolated and purified newly formed composition, determined its structure, and simulated reaction process to discover how the composition is formed. Results:One distinctive composition - 5-hydroxymethyl-2-furaldehyde (5-HMF)-was identified after processing, which is the product of Polysaccharides and organic acid (including Ferulic acid) reacting under high temperature.Conclusion:The distinctive composition of 5-HMF was for the first time, identified and isolated from Dangshen’s processed product. With deep analysis of reaction process, this study provides evidences for further research on processing principle of Dangshen.