A novel acid polysaccharide (PPRLMF-2) with the Mw of 137,123 Da and a triple-helix conformation was first isolated from the pulp of Rosa laevigata Michx fruit. Structural characterization showed that PPRLMF-2 consisted of rhamnose (7.6%), arabinose (26.5%), xylose (3.5%), mannose (0.9%), glucose (5.7%), galactose (31.9%) and galacturonic acid (23.9%). The methylation and NMR (1D and 2D) analysis revealed that PPRLMF-2 contained 16 types of glycosidic linkages. The immunomodulatory activity assays indicated that PPRLMF-2 could significantly enhance phagocytosis, the secretion and mRNA expression of cytokines in RAW 264.7 cells. Additionally, SR, GR, TLR-2, and TLR-4 were the main pattern recognition receptors (PRRs) of PPRLMF-2 to upregulate the p-ERK, p-JNK, p-p38, and p-p65. These results suggested that PPRLMF-2 could recognize the PRRs of the macrophages to enhance the immunomodulatory activity via activation of the MAPKs and NF-κB signaling pathways. This study provides important implications of PPRLMF-2 as an attractive immunomodulatory functional food.