Abstract: To study the quality formation mechanism of yellow tea, the main quality ingredients, polyphenol oxidase, peroxidase, catalase and cellulose activities during the piling process were studied.[Method] The main national seed mingshan baihao was selected as the test object, by fresh leaves→fixing (pot temperature 230 ℃, cast leaves speed 4min)→ piling 1 h →piling 2 h→piling 4 h→piling 6 h→spreading→drying. Then sampled in order to measure the main quality ingredients,polyphenol oxidase, peroxidase, catalase and cellulose activities.S[Result]During the piling process, the contents of tea polyphel and catechin was decreased. TheScontents of flavonoids and theabrownins was increased, while the theaflavins was decreased after increasing trend. The contents of amino acid and protein was increased. The content of soluble sugar was decreased after increasing trend. The activity of Polyphenol oxidase, catalase and cellulase was increased, the difference was significant. The peroxidase activity was decreased after increasing trend, the activity was the highest while piling 2 h, the difference was significant. [Conclusion] During the piling process, the quality ingredients are significantly changed. The contents of tea polyphel and catechin was decreased, which provides an important material foundation for the unique quality formation of yellow tea: mellow taste. Polyphenol oxidase, peroxidase, catalase and cellulose activities are close to zero after fixing, then has a certain degree of ‘Resurrection’ during the piling process, which acts together with heat involved in the formation of yellow tea quality.