Abstract: In the back-projection process of the FDK algorithm,interpolation is an important step, which affects the speed and quality of reconstructed image directly. Based on the detector’s structure, this paper gives a new interpolation method. This method gets the length that the ray passed through the voxel fast, and considers that the distance between the ray and the central of voxel has an effect on the weight, thus makes the value of weight more accurate. The experimental results show that the FDK algorithm of this new interpolation method gives a better result in the verge of the reconstructed image than the traditional interpolation methods, and this new interpolation method can restrain noise effect better.