Hydroxyapatite coating was prepared by alternate soaking strategy in short period time and ambient condition. According to the pH near equality between chitosan acidic acid solution and H2PO4 - aqueous solution, we proposed to choose H2PO4 - as one of hydroxyapatite precursor instead of traditional candidate (HPO4 2-). The properties of chitosan hydrogel containing plenty of water provide enough spaces for hydroxyapatite precursor to diffuse into framework spontaneously. XRD, FTIR and SEM were used to characterize the component and microstructure of mineralized chitosan. The ageing process helps to transfer amorphous calcium phosphate in chitosan framework into hydroxyapatite. The hydroxyapatite coating is gradient structure according to the result of SEM.