The phthaloyl polysaccharide (PHEP-2a) was prepared from a Lachnum exopolysaccharide (LEP-2a) by chemical modification, with a degree of substitution of 0.251. The results of UV, FT-IR, H-1 NMR and C-13 NMR analysis indicated that phthaloyl modification of polysaccharide was successful. Moreover, both elemental contents and thermal properties of the polysaccharide were changed after phthaloyl modification. Immunomodulatory activity evaluation revealed that compared with negative control group, LEP-2a and its derivative at high dose could remarkably increase the body weight, organ indexes, phagocytic index and half hemolysis value, promote Splenocytes proliferation activity, improve the production of serum cytokines, such as interleukin-2 and tumor necrosis factor-alpha (TNF-alpha), and enhance the activities of acid phosphatase and lactate dehydrogenase in cyclophosphamide-treated mice. They could also enhance the contents of T-SOD and GSH-Px while decrease MDA levels in cyclophosphamide-treated mice. Furthermore, PHEP-2a showed more significant effects than LEP-2a at the same dosage, indicating that phthaloyl modification was an effective way to enhance the immunomodulatory activity of the polysaccharide. (C) 2016 Published by Elsevier Ltd.