To study the condensation mechanism of sodium new houttuyfonate, and determinate the chemical structure of condensation products, dimer was prepared, and LC-DAD-MS/MS multiple techniques were    employed to investigate the ultraviolet absorption feature and mass spectrum of transformation solution of   dimer, and the transformation kinetics and half-life were studied by ultraviolet spectrophotometry.  The pure substance of stable condensation product was obtained by extracting with organic solvent and purifying with column  chromatography, the chemical structure of this substance was identified by assaying of IR, HR-ESI-MS and NMR, and the data of LC-MS/MS were compared with that of transformation products of dimer.  The    results indicated that the dimer is unstable, it will be rapidly dissociated in aqueous solution to form free new houttuyfonate and then cycloaddition reaction will occur and followed by an in situ dehydration to generate 1, 3, 5-tri (dodecanoyl) benzene (trimer) with a six-ring which is stable in aqueous solution.  The transformation process may fit second-order kinetics, and the half-times were found to be 3.17 hours at 25 ℃ (298 K) and 6.39 min at 100 ℃ (373 K), separately.  It suggests that dimer is an intermediate in condensation reaction, and the end condensation product of sodium new houttuyfonate injection may exist as trimer.