To ameliorate the serious sidelobe noise and high time complexity of the conventional synthetic-aperture algorithm in medical ultrasonic endoscopy systems, a coherence-factor-based real-time synthetic-aperture algorithm is proposed in this paper, which is realized with the high-speed parallel processing capabilities of a graphic processor (GPU). Introducing the coherence factor effectively suppresses sidelobe noise in an ultrasonic echo image, and the algorithm is processed using a GPU to improve its speed. The simulation data and the actual data are compared for experimental verification, and the proposed algorithm is shown to effectively improve the quality of the ultrasonic echo image. When the echo image size is 153 Mbit, the operation speed is 16.29 times faster than that obtained by using the conventional synthetic-aperture algorithm. A processing speed of 6 frame·s -1 is achieved to satisfy the requirements of real-time processing for medical ultrasonic endoscopy.