The potential immunostimulatory effects of Astralagus membranaceus polysaccharides (APS) on sea cucumber, Apostichopus japonicus (Selenka), were investigated in vitro. Phagocytosis and superoxide anion (O 2 − ) production by phagocytic amoebocytes (PA) from A. japonicus coelomic fluid were measured during incubation at 18 °C, 22 °C, or 25 °C with APS at 0, 10, 20, or 40 μg mL −1 (n=3). Phagocytic activity against yeast cells was quantified by direct visualization, and O 2 − production by nitroblue tetrazolium (NBT) reduction assay. Compared with controls, including APS at 20 μg mL −1 significantly increased (P<0.05) the percentage of phagocytic capacity (PC) and phagocytic index (PI) at 18 °C and 22 °C, but no significant enhancement was observed at 25 °C. In contrast, the coelmocytes of A. japonicus can have an obvious generation of O 2 − after the stimulation. The concentration of 20 μg mL −1 APS resulted in a significant increase in nitroblue tetrazolium (NBT) positive cells (P<0.05) at different temperature and even 10 μg mL −1 APS could increase O 2 − generation significantly at 18 °C and 22 °C. Both phagocytosing and O 2 − production increased with the increase of APS concentration from 0 to 20 μg mL −1 at different temperature, and when APS at 40 μg mL −1 , they were decreased. It suggested that immunocytes activity in A. japonicus decreased with the temperature increasing from 18 °C to 25 °C, and APS could be an effective immunostimulant to enhance phagocytic activity and O 2 − production.