Background: YiqiHuoxue decoction (YHD) is frequently prescribed to prevent and treat cardiovascular diseases. YHD inhibits platelet aggregation, however the underlying mechanisms are unclear. Methods: The in vitro and in vivo anti-platelet and antithrombotic effects of YHD and ethanol-precipitated YHD (EYHD) and underlying mechanisms were investigated. Forty-six Sprague-Dawley (SD) rats and 36 male Kunming mice were examined. Ten SD rats were used to assess the cytotoxicity of YHD and EYHD by releasing lactate dehydrogenase from treated platelets. The remaining 36 SD rats were divided into six groups (six per group), including control saline (5 mL/kg), aspirin (20 mg/kg), YHD low dosage (0.2 g/kg), YHD high dosage (2.0 g/kg), 75% EYHD low dosage (0.2 g/kg), and 75% EYHD high dosage (2.0 g/kg) groups to detect platelet aggregation; the 36 Kunming mice were divided into 6 groups to detect mesenteric arterial thrombosis induction. Thromboxane B2 (TXB2) levels were determined by enzyme immunoassay. Results: YHD high dosage and 75% EYHD (low and high dosage) inhibited ADP-induced platelet aggregation. Moreover, collagen-induced platelet aggregation was significantly suppressed by YHD (high dosage), 75% EYHD (high dosage), and 75% EYHD (low dose). Rats given 75% EYHD (high dose) displayed a marked reduction in collagen-induced platelet aggregation at 2 h post-administration. YHD and EYHD markedly prolonged the onset of thrombosis causing loose attachment of the thrombus to the vascular endothelium, but bleeding and clotting times were not significantly changed. Finally, YHD and EYHD markedly reduced TXB2 levels. Conclusions: YHD and EYHD effectively inhibit platelet activation and thrombosis, presumably by suppressing TXB2.