Dendrobium devonianum, a precious species of Dendrobium, is also called “Zi-pi Shi-hu” in Chinese due to its purple-skinned appearance. In this study, polysaccharides from green (GDP) and purple (PDP) skin D. devonianum were isolated by homogenate assisted extraction, ethanol precipitation and ultrafiltration. Their difference in physicochemical properties and immunological activities were investigated and compared. The results suggested that both GDP and PDP mainly contained glycosidic linkages of β-1,4-Manp and β-1,4-Glcp, but their molecular weights, 2.044 × 10 (GDP) vs 3.563 × 10 (PDP), and compositional monosaccharides of mannose to glucose ratio, 16.46: 1.00 (GDP) vs 7.61: 1.00 (PDP), were significantly different. Especially, GDP showed stronger immunological potentiation activity in vitro. Although the extraction yield of GDP was lower than that of PDP, considering the production of raw material and immunological activity of its polysaccharides, green skin D. devonianum should be a good alternative of purple one. These findings are helpful to provide evidence for improving the cultivation and production of D. devonianum.