Aim at traditional ray casting algorithm existing the problems that powerful computing capacity and slow rendering velocity, an improved ray casting algorithm is presented in this paper. To reduce the number of voxels in reconstruction process, the image segmentation is introduced to volume rendering. A segmentation algorithm based on multiple structure elements is proposed to process ICT slice images original data field. The original voxels are divided into two parts: the target voxel data set and the background voxel data set. Then, the target voxel data set is constructed as a new data field, and using the ray casting algorithm to it. This reduces the number of voxels and accelerate the efficiency of ray casting. The results of reconstruction show that this algorithm can greatly reduce reconstruction volume data and improve the efficiency of ray casting.