The selective laser sintering (SLS) technology is applied to the molding of the graphite/phenolic resin mixed powders, the change rule of the dimensional precision of the graphite prototype is studied experimentally, and its internal cause is revealed. It is found that the dimensional deviations in both the X and Y directions of the graphite prototype increase with the increase of the laser energy density, and decrease with the increase of the layer thickness. The dimensional deviation in the Z direction shows a tendency to decrease first and then increase with the increase of the laser energy density and the layer thickness. Compared with the dimensions in the X and Y directions, the dimension in the Z direction is more obviously affected by the process parameters. The main reasons for the large dimensional deviation of the graphite prototype are the secondary sintering and the Z-axis bonus, which are produced in the SLS process. The orthogonal experimental results show that the factors affecting the relative dimension error in the Z direction are sorted by the order of influence degree from large to small as the scanning interval, the layer thickness, the laser power and the scanning speed. The optimal combination of process parameters which makes the relative error in the Z direction smallest is: laser power of 23 W, scanning speed of 1000 mm·s-1, scanning interval of 0.18 mm and layer thickness of 0.14 mm.