This study optimized the hot water extraction (at 60 ℃ for 2 h) of arabinogalactan from larch (Larix spp.) and purified it using XAD-16N resin. High-purity (93.79%) water-soluble arabinogalactan (AGR) was obtained and its structural characteristics were evaluated. After its Smith degradation, five products including DAG1-10, composed of arabinose and galactose at a mass ratio of 1:11.15, were obtained. A negative correlation existed between the degradation time and the molecular masses of the products. The in vitro immunoregulatory evaluation indicated that DAG1-10 significantly promoted the secretion of immune mediators such as tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) by macrophages, thereby confirming its potential as an immunoenhancer. This research provides theoretical support for the high-value application of Larix spp. polysaccharides in the field of food science.