The invention discloses a thermoplastic starch and a preparation method thereof. The preparation method comprises the following steps: starch, glycerin or sorbitol, rosin glyceride and stearic acid are mixed with water; the solution reacts under the supercritical or subcritical state of carbon dioxide in an extruder; the reactant is extruded and chopped to obtain the particles of the thermoplastic starch. The thermoplastic starch of the invention can produce blow molding film and T film and can also adopt other thermoplastic-phrase polymer to carry out dry-mix production to produce the film with multilayered structure. The thermoplastic starch is miscibility with other polymers and can be used by combining general thermoplastic polymers such as polylactic acid, polyethylene, polypropylene, various PO polymers, daiamid, polyurethane, and the like, which can be molded by blow molding, injection molding and vacuum mold pressing, and the like in plastic (PO) mechanical processing. Furthermore, the thermoplastic starch is edible.