In order to provide reference for the quality improvement and process improvement of black tea in autumn, the fresh leaves of tea cultivars Qilan were processed to black tea in autumn, and the change law of content of main biochemical components, polyphenol oxidase (PPO) activity, and related enzyme gene expression was analyzed during processing process. The tea polyphenols, catechins, water extracts, and caffeine decreased from fresh leaves (XY) to the end of fermentation (FJ8) by 6.13%, 15.66%, 10.64%, and 0.10% respectively, and flavonoids, theaflavins, thearubigins, theabrownins, and free amino acids increased by 4.98 mg/g, 7.45 mg/g, 2.03%, 5.58%, and 0.14% respectively, among which the change range of caffeine was the smallest. Theaflavins, thearubigins, theabrownins and catechins showed a very significant negative correlation, and the correlation coefficient was -0.991, -0.969, -0.972 respectively. PPO enzyme activity fluctuated, increased first and then decreased in the withering stage, reached a peak value of 458.83 U/g during rolling, about 1.67 times that of XY, finally decreased rapidly in the fermentation stage, which was significantly lower than that of XY, and was about 0.89 times that of XY. The expression of polyphenol oxidase genes CsPPO1 and CsPPO2 both increased first and then decreased from rolling, and the highest relative expression appeared at 15 h of withering (WD15), 11.2 and 12.8 times that of XY, respectively. There was no significant difference between the CsPPOs expression in fermentation stage and fresh leaves. The expression of catechins metabolism related enzyme genes PAL, CHS, C4H, CHI, F3H, F3°5°H, FLS, DFR, LAR, and ANR was inhibited in varying degrees during processing, and significantly lower than that of XY. Except for the negative correlation with CG (Catechin Gallate), the genes were positively correlated with other components of catechins, and the correlation coefficient was 0.130-0.750.