Effect of acidizing conditions such as hydrochloric acid concentration, acidizing temperature and acidizing time etc. on moisture adsorption and desorption properties of sepiolite in closed space at constant temperature was studied. Relationships between moisture adsorption and desorption properties and microstructure of treated and untreated sepiolite were discussed by means of microscopic testing methods such as X-ray diffraction (XRD), Field emission scanning electron microscope (FESEM)and Brunauer-Emmett-Teller (BET). The results indicated that acidizing conditions had significant effect on moisture adsorption and desorption properties and microstructure of sepiolite in closed space. The optimum acidizing condition in this experimental program was obtained by treatment with 5 wt.% hydrochloric acid at 98 °C for 9 h. Fibers in the treated sepiolite become thinner and shorter, and carbonates in layers and pore canals were dissolved. Specific surface of sepiolite treated with the optimum acidizing condition increases from 35.64 m2/g to 84.86 m2/g ,and its average pore diameter decreases from 16.84 nm to 12.68 nm.