Flowers of Carthamus tinctorius L. are traditionally used in China to treat cerebrovascular and cardiovascular diseases. Hydroxysafflor yellow A (HSYA), the main constituent of Carthamus tinctorius L. flowers, is known for its multiple biological activities. In the present study, HSYA was isolated from Carthamus tinctorius L. flowers by a macroporous resin adsorption chromatography method coupled with a Waters high-throughput auto-purification system and it's vasodilatation effects on pulmonary artery (PA) were explored by an assay of tension study on rat pulmonary artery (PA) rings. Results suggest that HSYA possesses vascular relaxation effects on rat PA by activating the KV channel in pulmonary vascular smooth muscle cells (PVSMCs).