Aim To study the effects of doeosahexecnoic acid （DHA） on hypoxic pulmonary vasoconstriction （HPV） and its mechanism. Methods Pulmonary arteries were cut by 2 to 3 mm for vascular rings. Following the vascular constriction induced by acute hypoxia, DHA was administrated and the relaxing forces were measured without endothelia. Using the whole cell patch-clamp technique in freshly isolated pulmonary artery smooth muscle cells, the effects of DHA on total K^＋ currents, the large conductance Ca^2＋-activated K^＋ （BK_Ca） and voltage gated K^＋ currents were recorded. Results DHA relaxed vascular ring tone without endothelia （P<0.05）. DHA could significantly activate total K^＋ currents in pulmonary artery smooth muscle cells （P<0.01）. It also greatly enhanced BK_Ca currents （P<0.05） and at a testing potential of ＋60 mV, 125.21%±5.62%of BK_Ca currents were increased. In contrast, DHA inhibited K^＋ currents and at a testing potential of ＋60 mV, 63.21%±7.32% of K^＋ currents were decreased. Conclusion DHA can activate BK_Ca channels and relax the vascular constriction induced by acute hypoxia.