The quality of the green tea made from purple tea leaf, which is common in tea production, is not so good for its evident bitterness and astringency. In this paper, the main bitterness and astringency characteristic components of purple tea leaf were separated and determined, and the taste characteristics of those components were quantitatively evaluated by the threshold and index of bitterness and astringency. The results showed that the content of anthocyanidins was as high as 1.14%, and the contents of anthocyanidins and catechins of purple tea leaf were 57 and 1.13 times to those of green tea leaf, their higher content was mainly responsible to the bitterness and astringency of purple tea leaves. The order of threshold of those components was caffeine (0.30) < anthocyanidins (0.40) < catechins (0.80), and the order of bitterness and astringency index was catechins (3.32) > caffeine (2.13) > anthocyanidins (0.57), which was also used to evaluate the effect of components on the taste of tea. Furthermore, adding 0.2% casein into purple tea leaf during green tea processing could reduce the bitterness and astringency, the quality could be promoted, the concentration of tea polyphenols in tea infusion could reduce 19.5%, and the bitterness and astringency index decreased from 3.70 to 2.98. The research provided a technological method to reduce the bitterness and astringency of purple tea leaves. By using the polyphenol-protein complex reaction model, the relative precipitation rate (R_A) was used as the index to evaluate the inhibiting ability of casein to the bitterness and astringency of polyphenols.