In order to reveal the influencing mechanism of ultrasonic on the antioxidation defense system of microorganisms,catalase（CAT） was treated with ultrasonic and the effects of ultrasonic parameters on the enzyme activity of CAT were studied.Moreover,the secondary structure of CAT with and without ultrasonic treatment were analyzed by means of FT-IR.The results show that（1） the enzyme activity of CAT decreases with the increase in ultrasonic power;（2）the prolonging of irradiation time has no obvious effect on the enzyme activity of CAT;（3） the stable temperature and pH value for CAT treated with ultrasonic are respectively 37℃ and 7.4;and（4） after ultrasonic treatment,the kinetic parameters Km and Vmax of CAT both increase,and the secondary structure transfers from an ordered structure to a disordered one.