In order to investigate the substance transformation of polyphenols in black tea before and after processing， ultra-performance liquid chromatography-high resolution mass spectrometry combined with mass spectrometry molecular network were used to analyze the tea samples before and after processing. In this study， a total of 250 polyphenolic compounds were detected from tea samples before and after processing， including 115 catechins， 62 phenolic acids and 73 flavonoids. Among the 201 compounds， 174 compounds showed significant differences in content (P<0.01). 49 compounds were newly generated after black tea processing， among which 21 were theaflavins.