Abstract: Rapid resolution liquid chromatography coupled with quadrupole-time-of-flight mass spectrometry (RRLC-Q-TOF MS) method was developed for analyzing the hydrolysis of ginsenoside R_d in acidic condition. The hydrolysis products were identified by comparison with the retention time of the standard compounds, the accurate mass and the characteristic fragment ions obtained from RRLC-Q-TOF-MS/MS analysis, including C-Y₁, C-Y₂, F2, 20(S)-R_g3, 20(R)-R_g3, R_k1 and R_g5. The specific product ions of aglycone PPD (m/z 459), Δ20 (21)-or Δ20 (22)-dehydration-PPD (m/z 441) and C-24, C-25 hydratedPPD (m/z 477) by MS/MS were discussed for structural characterization. The chemical transformation mechanisms of ginsenoside Rd were demonstrated in acidic condition， include saccharide hydrolysis, Δ20(21)- or Δ20(22)-dehydration and hydration addition reactions at C-24, C-25. The hydrolysis transformation pathway of ginsenoside R_d was summarized. The developed RRLC-Q-TOF-MS/MS method was applied for comparative analysis of ginsenoside R_d and related hydrolysis transformation products.