The ray casting is a widely used basic volume rendering algorithm. It can get high quality image but the computation of traditional sampling-points method is overlarge and the rendering speed is very slow. Aimed at the shortcoming, an algorithm that takes the advantage of the recurrence relation of parallel projection line during the sampling process is presented to improve the speed of obtaining the sampling-points and the speed of reconstruction. Finally, the algorithm is realized on PC platform. The result shows that compared with standard ray casting algorithm, the accelerate algorithm does not only improve the rendering speed for nearly ten times but it does not reduce any image quality. It gives a more capable method for the application of medical image 3D reconstruction.