By treating chitin with ultrasonic in hot concentrated NaOH solution for about 30-120min, chitosan swellable highly in alkali was prepared. The swelling behavior and the structure of the prepared chitosan were invested. It was shown that the prepared chitosan can swell highly in NaOH solution by freeze-thawing treatment, with the particles turns into hydrogel. The swelling behavior showed that with the increase of NaOH concentration, the swelling degree of chitosan increases, until a concentration of 8.0 wt%, and ultrasonic treatment time has less significant influence on the swelling behavior. SEM showed that ultrasonic treatment makes the structure of chitin enough looser for the permeation of NaOH and H2O, resulting in the swelling of chitosan in alkali solution.