Abstract: Taking the tree bark of Pinus kesiya var.langbianensis as raw material, sulfurous acid as chemical reagent to degrade procyanidins from the bark to proanthocyanidins, the influence of the extraction times, the duration, temperature, solidliquid ratio and the reagent dosage for the extraction upon the proanthocyanidins degradation was studied. The results showed that each factor had a certain effect on the proanthocyanidins extraction. The comprehensive analyses indicated that the optimal parameters for the extraction of proanthocyanidins from the tree bark were 3 times of extraction under 90℃ with 1∶10(g/mL) of solidliquid ratio, 22% of sulfite in total liquid, and each extraction time duration was 100min. With such technology, the extraction rate of proanthocyanidins reached 752%, and their average polymerization degree was reduced to 340.