This paper reports on a water-soluble acid polysaccharide (AAP) and in how it was extracted from Auricularia auricular, acquired by CTAB, and prepared it’s carboxymethylation. Chemical characterization by high-performance liquid chromatography/gel permeation chromatograph (HPLC/GPC), Fourier transform infrared (FT-IR) spectrometer and gas chromatography–mass spectrophotometer (GC–MS) were investigated. Chemical analysis indicated that C AAAP was composed of arabinose, xylose, mannose, glucose and galactose, with the molar ratio at 0.04: 0.13: 1.00: 0.59: 0.29. Moreover, radiation protection against UVB in vitro indicated that at the dose range of 200–500 μg/mL, C AAAP enhanced the protection of HepG2 cells against UVB cytotoxicity than AAAP. However, but at the dose range of 50–150 μg/mL the result was just opposite.