Catechins are the key substances that endow green tea with a bitter and astringent taste, and the changes in their structures and content have an important effect on the flavor quality of green tea. By collating the current research results related to catechins, this paper summarizes the changes of catechins during the main processing steps including withering, de-enzyming, rolling, and drying of green tea, and explores the main mechanisms of changes in catechins during processing and their effects on the bitterness and astringency of green tea.