Chaenomeles sinensis fruit polysaccharide (CSP) and carboxymethylated CSP (CSP-M) were prepared using ultrasound extraction and the sodium hydroxide-chloroacetic acid method. Structural analysis revealed that both CSP and CSP-M mainly consisted of glucose, arabinose, rhamnose, glucuronic acid, galactose, and xylose, and the introduction of carboxymethyl did not damage the polymer chain of CSP. In vivo studies verified that both CSP and CSP-M could remarkably alleviate the symptoms of ulcerative colitis (UC) mice and reduce intestinal epithelial cell depletion, along with the infiltration of inflammatory cells in colon tissue, by mediating the expression of myeloperoxidase (MPO), inflammatory factors [tumour necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and IL-6], and oxidative stress factors [malondialdehyde (MDA), superoxide dismutase (SOD), glutathione (GSH), and nitric oxide (NO)]. Most importantly, the introduction of carboxymethyl significantly enhanced the anti-UC activity of CSP, confirming the efficacy of carboxymethylation as a method to enhance the biological activities of CSP, thereby suggesting the potential of CSP-M as a therapeutic option for UC.