Lanzhou lily (Lilium davidii var. unicolor Salisb.), a medicinal and edible plant found in China, is commonly referred to as the sweet lily. To further investigate the prebiotic potential of lily polysaccharides (LLP), one of its major bioactive components, and enhance the utilization of lily resources, ultrasonic-assisted enzymatic extraction with volume fractions of 60%, 80%, and 95% ethanol was conducted to obtain three types of LLP. Their in vitro digestive tolerance and prebiotic activity were then examined. Results demonstrated that as the ethanol volume fraction increased, the soluble total sugar content, extraction yield, protein content, and tolerance to simulated gastric and intestinal fluids of Lanzhou LLP improved. Notably, LLP-95% exhibited a soluble total sugar content and extraction yield of 19.83% and 23.77%, respectively, with a hydrolysis degree as low as 5.90% in simulated gastric fluid after 6 h. Additionally, the protein content was relatively low (4.09%). When LLP-95% was used as a carbon source, Lactobacillus plantarum fermentation for 48 h resulted in a prebiotic index of 0.96, pH of 4.73, and OD600 of 0.75, significantly outperforming fructooligosaccharides and the other polysaccharide groups. Furthermore, after a 40-min incubation in simulated gastric and intestinal fluids, the survival rates of LLP-95%-treated bacterial suspensions were 25.89% and 56.00%, respectively, which were significantly higher than those of the other groups. This study demonstrated that LLP-95% possesses good digestive tolerance, promotes the proliferation of L. plantarum, and enhances bacterial survival in simulated gastrointestinal environments, indicating a strong prebiotic potential. These findings provide scientific evidence for the development of functional foods and pharmaceuticals.