The aim of this study is to investigate the effects of Brassica rapa L. acidic polysachharide-1 (BRAP-1) on type 2 diabetes mellitus and its mechanism. The mouse model of type 2 diabetes mellitus was established by high-fat diet combined with streptozotocin induction. Mice were randomly divided into a control group, a model group, a metformin group (200 mg/kg), a low-dose BRAP-1 group (50 mg/kg), a medium-dose BRAP-1 group (100 mg/kg) and a high-dose BRAP-1 group (200 mg/kg).The content of total cholesterol (TC), triglyceride (TG), high density lipoprotein cholesterol (HDL-C), low density lipoprotein cholesterol (LDL-C), glycated serum proteins (GSP), insulin (INS) in serum and superoxide dismutase (SOD), malondialdehyde (MDA) and glutathione (GSH) in the colon were detected by biochemical kits. The levels of lipopolysaccharide, D-lactic acid, diamine oxidase in serum and tumour necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), IL-1β, GSH, SOD and MDA in colon were detected by ELISA.The pathological changes of colon tissue in mice were observed by HE staining. The protein expression levels of zonula occludens-1 (ZO-1), Claudin1, Occludin and myeloid differentiation primary response protein 88 (MyD88)/Toll-like receptor 4 (TLR4)/nuclear factor kappa-B (NF-κB) pathway proteins in colon were detected by western blot. The results showed that BRAP-1 reduced the serum levels of TC, TG, LDL-C, GSP and INS(P < 0.05 or P < 0.01), while increasing HDL-C level (P < 0.05) . It also elevated the levels of SOD and GSH in serum and colon, decreased the levels of TNF-α, IL-6, IL-1β and MDA in serum and colon (P < 0.05 or P < 0.01). Additionally, BRAP-1 upregulated the protein expression of intestinal barrier (ZO-1, Claudin1, Occludin) and downregulated the protein expression of MyD88, TLR4 and NF-κB p65 (P < 0.05 or P < 0.01). In conclusion, BRAP-1 can regulate glucose and lipid metabolism, alleviate endotoxemia, reduce intestinal inflammation and oxidative damage, protect intestinal barrier function and improve type 2 diabetes mellitus by inhibiting the MyD88/TLR4/NF-κB pathway.