Abstract: The study prepared Paecilomyces hepiali mycelium polysaccharide-stabilized selenium nanoparticles (SeNPs) and explored its potential to promote bone formation. A polysaccharide fraction (PHPW) was isolated and purified from P. hepiali mycelium water extract using dialysis, alcohol precipitation, deproteinization, and anion exchange column chromatograph. The physicochemical properties of PHPW were analyzed using the phenol-sulfuric acid method, high-performance gel permeation chromatograph, and high-performance anion exchange chromatograph. Subsequently, PHPW was used as a template to prepare PHPW-SeNPs using an oxidation-reduction method. The structural properties of the PHPW-SeNPs were characterized by nanoparticle size analyzer, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy, and its storage stability was further investigated. Subsequently, the effects of PHPW-SeNPs on MC3T3-E1 cells proliferation and differentiation were evaluated using the Cell Counting Kit-8 (CCK-8) assay, alkaline phosphatase (ALP) assay, and real-time quantitative PCR. The results showed that PHPW was a neutral polysaccharide primarily composed of glucose, and its polysaccharide content and weight-average molecular weight were 99.1% and 29.4 kDa, respectively. PHPW-SeNPs prepared using 200 μg/mL of PHPW exhibited the smallest particle size and good dispersion, presenting zero-valent, amorphous, and spherical structures. Among the analyzed elements, carbon, oxygen, and selenium accounted for 63.1%, 3.5%, and 33.4% of the total, respectively. The SeNPs were stabilized by the hydroxyl groups in PHPW through hydrogen bond-like interactions. The stability of the PHPW-SeNPs solution was observed to exhibit good stability at a pH value range of 6~8 and in the dark at 4 ℃. The PHPW-SeNPs increased the proliferation rate of MC3T3-E1 cells in a dose- and time-dependent manner at a concentration range of 1.25~20 μmol/L. Compared with the control group, PHPW-SeNPs at 20 μmol/L significantly enhanced the ALP activity of MC3T3-E1 cells by 1.8-fold. Moreover, the mRNA expression levels of bone morphogenetic protein 2 (BMP2), Runt-related transcription factor 2 (RUNX2), osteorix (OSX), and ALP were up-regulated by 2.9-, 4.7-, 3.2-, and 3.8-fold, respectively. In conclusion, stable PHPW-SeNPs was successfully prepared using PHPW as a template. PHPW-SeNPs significantly promoted the proliferation and differentiation of MC3T3-E1 cells, showed good application potential in preventing osteoporosis, and could be developed into a health product to promote bone health.