In order to obtain clear wheat grain reconstruction slices, the conjugated ray is used to compensate for the missing data of Radon space. Based on the FDK (Feldkamp-Davis-Kress) reconstruction algorithm, the z-axis weight function is introduced to optimize the reconstruction results of the grain slice images. For the three-dimensional head model, the root mean squared error of the reconstruction result by the z-FDK algorithm is 3.6927 smaller than that of the FDK algorithm and the z-axis strength drop in the FDK algorithm is greatly suppressed. As for the projection images of wheat grains at egg stage, young larva stage and old larva stage, the experimental results show that the average gradient and the contrast noise ratio of the z-FDK algorithm are both larger than those of the FDK algorithm. The gray values of the reconstructed two regions of the hair end and the embryo end of wheat grains increase and the artifacts are improved.