Objective To explore the mechanisms of Buyang Huanwu Decoction (BYHWD) in reducing oxidative stress levels to protect the blood-brain barrier (BBB) in cerebral ischemia / reperfusion injury (CIRI) rats. Methods A middle cerebral artery occlusion / reperfusion ( MCAO/ R) model in rats was established via wire embolization method .PeriCam PSI laser speckle flow imaging was applied to detect whether the model was successfully established. Neurological deficits in the rats were evaluated by Zea Longa score, and histopathological changes in the rat brain were observed by HE staining. The degree of brain edema was detected by the dry and wet weight method. BBB permeability was detected by Evans blue staining, and ultrastructural changes to the BBB were observed by transmission electron microscopy. The levels of ROS, MDA and SOD activities, which are related to oxidative stress, were detected using kits. The expression levels of matrix metalloproteinase-9 (MMP-9) were detected by immunohistochemical staining and Western blot. The expression levels of Occludin, ZO-1, and Claudin-5 tight junction proteins were determined via immunofluorescence and Western blot. Results BYHWD reduced neurological deficit scores, alleviated brain histopathological damage, alleviated BBB structural disruption, prolonged the appearance of dense regions in the tight junction structure, attenuated edema of the brain on the ischemic side, and reduced BBB permeability in MCAO/ R rats. BYHWD decreased the levels of ROS and MDA,increased the activity of SOD, decreased the expression levels of MMP-9, and increased the expression levels of Occludin,Claudin-5 and ZO-1. Conclusions BYHWD can increase BBB tight junction protein expression levels, reduce the permeability of the BBB, protect the ultrastructure of the BBB, and reduce brain edema, and its mechanisms may be related to its antioxidant activity and inhibition of MMP-9 activation.