Hepatic fibrosis (HF) arises from dysregulated wound healing during chronic liver injury, progressing to liver dysfunction and carcinogenesis. Polysaccharides have emerged as promising candidates for HF treatment. In this study, a homogeneous galactomannan (45.0 kDa) was isolated from the seeds of Astragalus complanatus R. Br. Structural analysis revealed that it was composed of T-galactose (Gal), 1,4-mannose (Man), and 1,4,6-Man in a molar ratio of 1.47:1.00:1.42, featuring a β-1,4-Manp backbone with terminal α-Galp branches at O-6 of Manp. Additionally, ACSP-I's triple-helical structure was confirmed by Congo red assay and circular dichroism spectrum, while scanning electron microscopy revealed a lamellar morphology with pores on a slightly rough surface. ACSP-I exerted potent anti-fibrotic effects, suppressing hepatic stellate cells (HSCs) activation in vitro and attenuating CCl4-induced liver injury in vivo. Mechanistically, ACSP-I suppressed HSCs activation through inhibition of pyruvate kinase M2 (PKM2)-mediated glycolysis, which suppressed the expression of proliferation-related genes (MYC and CCND1), and inhibited histone lactylation to downregulate fibrotic genes (ACTA2 and COL1A1). These results identify ACSP-I as a PKM2 inhibitor for HF treatment and reveal new mechanisms of polysaccharide-mediated hepatoprotection.