Two norditerpenoids (1–2) with unique carbon skeletons, four sesquiterpenoids (3–6) and nine nor-sesquiterpenoids (7–15) were isolated from the leaves of Crataegus pinnatifida and evaluated as possessing antithrombotic activities in vitro/vivo. Their structures with absolute configurations were determined via a combination of spectroscopic data, chemical methods, and quantum-chemical calculations (ECD, NMR, and OR data). Compound 3 showed an inhibitory effect on ADP induced platelet aggregation in vitro, which is mediated through the response to the specific receptor of P2Y₁₂ by docking results. Compound 3 also clearly prolonged the time to form thrombocytes induced by FeCl₃, in the caudal vessels of zebrafish.