Objective： In this study， the polysaccharide derived from Bangia fusco-purpurea was separated and purified， and its chemical composition and in vitro immune-induced activity were analyzed to illuminate food and medicine mechanism of Bangia fusco-purpurea， which provided scientific basis for the high-efficient utilization of Bangia fusco-purpurea. Methods： Crude polysaccharide was extracted from Bangia fusco-purpurea by hot water extraction and ethanol precipitation. The polysaccharide fractions （F1， F2 and F3） were obtained with isolating and purifying polysaccharide BFP by cation exchange column chromatography， Sephadex G75 chromatography and DEAE-cellulose 52 exchange chromatography. The monosaccharide composition and chemical component of BFP， F1， F2 and F3 were analyzed by PMP pre-column derivatization high performance liquid chromatography （HPLC） and the chemical methods. Further， the in vitro immune-induced activity and signal transduction pathway of BFP， F1， F2 and F3 were analyzed by mouse macrophage RAW264.7 cells model. Results： The results of chemical component and monosaccharide composition showed that BFP， F1， F2 and F3 were heteropolysaccharides with significant differences in monosaccharide compositions. The in vitro immune-induced activity cells model revealed that BFP， F1， F2 and F3 significantly induced RAW264.7 cells activation to release NO and TNF-α in the measured concentration range （0~100 μg/mL）， but did not induce the generation of ROS. Cellular signal pathway inhibition experimental results show that the immune-induced activity of BFP was related to the cellular signal transduction pathways activation of NF-κB， JNK MAPK， ERK MAPK， and p38 MAPK； F1 was associated with the cellular signal transduction pathways activation of NF-κB， JNK MAPK， ERK MAPK and p38 MAPK； however， the mechanisms of action of F2 and F3 were similar， which were related to the activation of NF-κB， JNK MAPK and ERK MAPK signal transduction. Conclusion： The BFP possessed strong in vitro immune-induced activity， of which fractional purified polysaccharide fractions F1 and F2 were the major immune-induced components. The common signal transduction pathways of BFP， F1， F2 and F3 induced RAW264.7 cells release NO were NF-κB and JNK MAPK. However， the common signal pathways inducing TNF-α secretion in RAW264.7 cells were mainly JNK and ERK MAPK signaling pathways.