Russula alutacea Fr. (RaF) is a tasty mushroom with high nutritional value and have been regarded as food by local people for a long time. However, few researches have focused on the polysaccharide in RaF. In this study, a purified polysaccharide (Rap-1) was isolated with an average molecular weight of 1029.7 kDa. In vitro, Rap-1 significantly suppressed cell morphological changes and inhibited the production of nitric oxide (NO) in LPS-induced RAW 264.7 cells extracellularly and intracellularly. Besides, Rap-1 also down-regulated the expression of nuclear factor kappa-B (NF-κB), inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX-2) contents. Rap-1 ameliorated the oxidative stress and mitochondrial dysfunction caused by LPS via MAPKs signaling pathways in macrophages. In vivo, Rap-1 decreased the ROS and O2- levels and recovered the heart in zebrafish (Danio rerio) larvae induced by LPS, These results together suggested that Rap-1 could be a potential functional resource to protect against inflammatory and oxidative damage.