Abstract: The gravity or gravity anomaly calculating methods must be developed from the rectangular Cartesion coordinate system to the spherical coordinate system when utilizing gravity data to study the regional and global geologic problems. The method of surface integral approximation for gravity anomaly forward in spherical cap is improved so that both calculation accuracy and computing speed are enhanced. By means of the model testing, the calculation accuracy and stability of the methods of gravity anomaly calculation in spherical cap and the Tesseroid-Taylor series expansion are analyzed, and the conclusions are drew that the Tesseroid-Taylor series expansion approach is affected by the expansion order, surface area, thickness, depth and surface position of the Tesseroid, and the height of the calculating point, while the method of surface integral approximation in spherical cap by the thickness and depth of the Tesseroid, and the height of the calculating point and the accuracy of model reconstruction, but the method proposed in this paper only by the model reconstruction. the Tesseroid-Taylor series expansion approach, which does not require coordinate rotation and model reconstruction, is fit for 3-D density imaging by gravity data, but the calculating method in the spherical cap, which has high accuracy and stability, is suite to local, regional and global gravity or gravity anomaly forward modelling.