Platelet hyperactivation could lead to various cardiovascular and cerebrovascular diseases, while epidemiological analyses have found that long-term tea drinking could prevent and restrain cardiovascular diseases. Existing studies have shown that catechins, especially epigallocatechin gallate (EGCG), are the main functional factors of tea in alleviating thrombosis, which could inhibit arterial thrombosis and platelet aggregation induced by a variety of agonists. However, their structure–activity relationship and the underlying mechanisms are still unclear. Based on the above background, this study took six typical catechins as research objects, constructed platelet activation models with different inducers, and explored the inhibitory effects and potential mechanisms of catechins with different structures on platelet aggregation through flow cytometry, immunoblotting, cell spreading, and other experiments. It was found that ester catechins could inhibit platelet aggregation induced by adenosine diphosphate (ADP), while epigallocatechin (EGC) with three hydroxyls on the B ring in non-ester catechins was also able to effectively inhibit platelet aggregation. Our data suggested that gallic acyl on the C ring and three hydroxyls on the B ring were the main functional groups affecting the antithrombotic effect of catechins, and the effect of gallic acyl on platelets was significantly stronger than that of the hydroxyl.