The aim of this study is to probe new functions of a polysaccharide from Spirulina platensis (PSP) on constipation and intestinal microbiota in mice. Diphenoxylate-induced constipation in mice was treated with different doses of PSP, followed by examining the defecation patterns, levels of acetyl cholinesterase (AchE), nitric oxide (NO), and tissue section histopathology. The composition of intestinal microbiota was determined by genome sequencing analysis of the 16S rDNA. This study found that the average molecular weight of PSP was 29, 600 Da, and mainly monosaccharides of PSP were rhamnose (24.7%), glucose (16.15%) and galactose (13.32%). The beneficial effects of PSP treatment include defecation improvement, increase of AchE activity, reduction of NO concentration, renovation of the damaged intestinal villus and affection on the expression of some related genes in the constipated mice. In addition, PSP had significant effects on the gut microbiota, showing the enhancement in abundance of beneficial bacteria including Akkermansia, Lactobacillus, Butyricimonas, Candidatus Arthromitus and Prevotella, and the reduction in abundance of harmful bacteria such as Clostridium and Dorea. The present s uncovered a new function of PSP, indicating that PSP could be used in constipation therapies.