The crude polysaccharides of Citrus aurantium L. var. amara Engl (CAVAPs) were extracted and their bioactivities including DPPH radical scavenging activity, cytotoxicity to human breast cancer cells, MCF-7, as well as lung cancer cells, HCC827, and their immune-enhancement activity were evaluated. Results showed that CAVAPs exhibited better immunoenhancement activity compared to the DPPH radical scavenging and anticancer activities. Subsequently, the immune enhancement activity of CAVAPs on RAW264.7 cells was further observed and the results displayed that CAVAPs could significantly stimulate the production of tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) in RAW264.7 cells, and promote the mRNA expression levels of inducible nitric oxide synthase (iNOS), TNF-alpha, interleukin-1 beta (IL-1 beta), and IL-6. Furthermore, western blot analysis demonstrated that the phosphorylated extracellular signal-regulated kinase (ERK), phosphorylated c-Jun N-terminal kinase (JNK), phosphorylated p38 and phosphorylated p65 were all remarkably increased in CAVAP-treated RAW264.7 cells. All these results indicated that CAVAPs might activate macrophages through the mitogen-activated protein kinase (MAPK) and nuclear factor-kappa B (NF-kappa B) signaling pathway. Additionally, a three-level-three-factor Box-Behnken design (BBD) was performed to optimize the extraction process of CAVAPs for the purpose of application and further research. The maximum extraction yield reached 4.49 +/- 0.25%.