This study evaluated the effects of dietary sodium butyrate (SB) and Artemisia seed polysaccharide (ASP) on growth performance, gastrointestinal development, and barrier function in 48 post-weaning Ordos fine wool lambs. Lambs were randomly divided into four groups (n = 12/group): control (CG), SB (0.5% SB), ASP (0.3% ASP), and SB+ASP. The findings showed that both SB and ASP significantly improved average daily gain (ADG) and spleen development. Notably, SB had a more substantial impact on foregut (rumen and abomasum) weight. Histological analysis revealed enhanced intestinal villus development in all treatment groups. Mechanistically, supplementation increased serum IGF-1 and GLP-2 levels, upregulated anti-inflammatory markers (NO,iNOS), and reduced inflammatory markers (CRP, SAA). Furthermore, Western blot analysis showed that SB and ASP increased the expression of intestinal tight junction proteins (Occludin, Claudin-1, ZO-1) in the small intestine. In conclusion, both SB and ASP enhance growth performance, and immunity in weaned lambs, likely via a GLP-2-mediated mechanism that strengthens the intestinal barrier. While SB excels in promoting rumen development, ASP offers a promising, sustainable option for improving gut health and performance in arid regions.