A three-dimensional volumetric display system was built based on two-dimensional LED array. Scan mapping method was used for data voxelizaion. Gray scale deviation was caused due to voxelization. Geometric approximation method for calculating gray scale deviation was introduced. Two groups of cube models were built and their mean gray scale deviations and root-mean-square deviations were calculated. The calculated results show that gray scale deviation affects display results more in those images that have more gray details. Display distortions may be caused in images that have plenty of gray details. Gray scale deviation could be corrected partly by improving the hardware of the system and by optimizing the voxelization algorithm. Simulating figures before and after gray scale deviation correction show that there are fewer stray points after deviation correction.