•Two derivatives, TAPA1-ac and TAPA1-deac, were prepared by chemical modification.•Chemical modifications were proved to occur by spectra and calculational results.•Immunostimulating activity of TAPA1, TAPA1-deac and TAPA1-ac, were determined.•The relationship between the group O-Ac and activities were preliminarily investigated. TAPA1 was previously isolated from Tremella aurantialba fruiting bodies. In this paper, an acetylated derivative (TAPA1-ac) and a deactylated derivative (TAPA1-deac) of TAPA1 were prepared, and their characterization and immunostimulating activities were reported. Acetylation and deacetylation were found to occur actually by FT-IR and NMR spectra, together with calculational results. The degree of substitution (DS) of acetyl groups in TAPA1-ac was 0.23 and the content was 5.82%, which was higher than those of TAPA1 (0.03% and 0.70%, respectively) and TAPA1-deac (all were zero). Compared with TAPA1, TAPA1-ac showed significant immunostimulation effects on mouse spleen lymphocytes (MSLs) proliferation and nitric oxide (NO) production by macrophages RAW264.7, whereas TAPA1-deac showed markedly lower effects. These findings seemed to suggest that immunostimulating activities, including MSLs stimulation activity and NO production potency, might relate to the DS and content of acetyl groups, indicating that acetylation of TAPA1 was an effective way of enhancing immuno-stimulating activities.