This paper presents a fast forward calculation method of gravity and magnetic anomalies for 3D arbitrary bodies based on GPU. The surface is subdivided into a large number of regular voxels. For any 3D arbitrary body, the total volume can be combined by these voxels with fixed size and unknown physical properties (density or susceptibility). The gravity (magnetic) anomaly of these voxels at the surface points can be computed by the analytical method. The sum of the above gravity (magnetic) anomaly of each voxel can represent the anomaly of the whole arbitrary body. For the purpose of improving the approximation, the subsurface should be divided into a larger number of voxels. The authors have adopted GPU parallel method for forward calculation because the traditional CPU program is time-consuming. The result shows that the forward calculation by GPU has correct results and improved efficiency. The idea can provide the basis for 3D inversion of gravity and magnetic anomaly.