Medical image visualization, also known as medical image 3D reconstruction technique, is a theory, method and technique which use computer graphics, image processing, computer vision and human-computer interaction techniques to reconstruct 3D stereoscopic image from the received 2D medical images (such as MRI, CT, etc.), then display it on-screen and interact with it. Currently, medical image 3D visualization technology is a hot in the medical image processing, and it is of great significance in theory research and clinical applications. This paper mainly introduces the key techniques in the medical image 3D visualization and designs a system of the medical image 3D visualization based on VTK. The system integrates Marching Cubes and Ray Casting visualization algorithm, implements the function of DICOM format of medical images file read, 3D reconstruction and display. The system can be applied to medical diagnosis, medical education and medical research, etc.