By sending a micro-ultrasonic probe into coelom through biopsy channel of electronic endoscope,ultrasonic endoscope system can not only acquire pictures of the alimentary canal mucosal surface by direct observing,but also get images of the full histology of the digestive organs by ultrasonic scanning. In this paper,we introduced the coded excitation technique into the ultrasonic imaging system,which uses a 4-bit Barker coded pulse to excite the ultrasonic transducer resulting that the detecting depth and imaging quality of the medical ultrasonic endoscope be increased greatly with the peak negative acoustic pressure unchanged. Then we utilized the digital down conversion technique to realize medium frequency sampling,pulse compression and envelope extraction of the ultrasonic echo. Finally,using a glass beaker with ultrasonic coupling liquid as experimental sample,achieved a 256 gray-scale ultrasound image with the axial resolution of 0.4 mm,so the results proved the validity of system principle.