ObjectiveTo explore the effect and mechanism of Sishenjian on synovial lesions induced by monosodium iodoacetate （MIA） in rats with knee osteoarthritis （KOA）. MethodSixty female Sprague-Dawley （SD） rats were randomly divided into the following six groups： normal group， model group， celecoxib group， and high-， medium-， and low-dose Sishenjian group. The KOA rat model was established by intra-articular injection of MIA. Celecoxib （18 mg·kg^-1） and Sishenjian （14.4， 7.2， 3.6 g·kg^-1） were administered by gavage according to the groups. All rats were euthanized after four weeks of continuous administration. The transverse diameter of the bilateral knee joints of rats was measured， and gross observation of the knee joint was performed. Pathological changes in knee joint synovial tissue were observed by hematoxylin-eosin （HE） staining and picrosirius red staining. Immunohistochemistry （IHC） was used to detect the expression of vascular endothelial growth factor A （VEGFA） in synovial tissue. The levels of inflammatory cytokines in the joint synovial fluid were detected by enzyme-linked immunosorbent assay （ELISA）. Real-time quantitative polymerase chain reaction （Real-time PCR） and Western blot were used to detect the expression of mRNA and proteins related to the transforming growth factor-β₁ （TGF-β₁）/Smad2/3 pathway in knee joint synovium. ResultCompared with the normal group， the transverse diameter of the knee joint in the model group significantly increased （P<0.01）. Compared with the model group， the transverse diameter of the knee joint in rats of each Sishenjian group significantly decreased （P<0.01）. Compared with the normal group， the expression levels of interleukin-1β （IL-1β） and tumor necrosis factor-α （TNF-α） in the knee joint synovial fluid of model group significantly increased （P<0.01）. Compared with the model group， the expression levels of IL-1β and TNF-α in the knee joint synovial fluid of rats in each Sishenjian group significantly decreased （P<0.01）. Compared with the normal group， the expression levels of TGF-β₁， Smad2/3， phosphorylation（p）-Smad2/3， type Ⅰ collagen α1 （ColⅠ_α1）， type Ⅲ collagen α1 （ColⅢ_α1）， VEGFA proteins and TGF-β₁， Smad2/3， ColⅠ_α1， ColⅢ_α1 mRNA in knee joint synovium of model group significantly increased （P<0.01）. Compared with the model group， the expression levels of TGF-β₁， Smad2/3， phosphorylation （p）-Smad2/3， ColⅠ_α1， ColⅢ_α1， VEGFA proteins and TGF-β₁， Smad2/3， ColⅠ_α1， ColⅢ_α1 mRNA in knee joint synovium of rats in each Sishenjian group significantly decreased （P<0.05， P<0.01）. ConclusionSishenjian can inhibit synovial inflammation and angiogenesis， and may become a potential drug for treating synovial lesions in KOA by regulating the TGF-β₁/Smad2/3 pathway.