Dynamic aperture control technology is one of the most important parts in ultrasonic imaging system. Excellent control technology of dynamic aperture can effectively improve the quality of ultrasonic imaging. Based on deep analysis of ultrasonic beam distribution, a control method for dynamic aperture which can eliminate the scanning dead zone of focused beam is presented. The detection region is even divided according to the number of probe array elements. The desired focused beam width is calculated between the next two scan lines at different depths, and then according to the beam width, the required aperture size for different depths is determined. The method is not affected by the probe parameters or detection objects, and it can be widely employed in ultrasonic imaging system. The simulation experiments indicate that the proposed method can effectively improve the whole quality of ultrasonic imaging.