Danshensu, the effective ingredient of the plant Salvia miltiorrhiza (Danshen), has been widely used for treatment of cardiovascular diseases. Cardiac fibrosis is an important process in pathological cardiac remodeling and leads to heart failure. We investigated the effect of Danshensu on β-adrenergic receptor (β-AR)-mediated cardiac fibrosis and the involved signaling transduction. Danshensu inhibited cardiofibroblast proliferation and collagen I synthesis induced by isoproterenol (ISO), a selective β-AR agonist. Phosphorylation of p38 mitogen-activated protein kinase (MAPK), which mediates ISO-induced cardiac fibrosis, was negatively regulated in this process. The negative regulation depended on the ISO inhibition of reactive oxygen species (ROS) production. Taken together, Danshensu may inhibit β-AR-mediated cardiac fibrosis by negative regulation of ROS-p38 MAPK signaling.