Abstract: This paper mainly studies the relationship between the effect of ultrasound on the enzyme activity of sodium alginate immobilized papain and the effect of ultrasound on the structure of alginate gels. On the basis of single factor experiment, the best ultrasound conditions were obtained through the uniform design test method (40 kHz, 0.45 W/cm², 20 min, 70℃), the predicted relative enzymatic activity was 342.64%. The impact of ultrasound on the structure of alginate gels was studied. Electron microscope and digital image processing technology were applied to makes a quantitative evaluation on gel structure. Scanning electron microscope (SEM) analysis exhibited that the gel structural area ratio changes were in accordance with activity through different ultrasonic treatment, which means under the condition of optimal ultrasonic enhancement, the gel structure changes most obviously. This result provided the evidence that ultrasonic treatment could raise the activity of sodium alginate immobilized papain, and there was a close relationship between structure of gel and activity of sodium alginate immobilized papain.