An approximate method for generating computer-generated holograms (CGH) of a 3D object with 6 times faster speed han the conventional algorithm is presented.In the conventional algorithm,a 3D object is sliced into many layers and treated as collection of self-illuminated point light sources.The propagation process of light ray from every points of object to all the points on the hologram plane is simulated and interfered with the reference beam to form a CGH.In our proposed method,under the assumption that the depth of 3D object is much smaller than the recording distance,we just need to calculate the oblique distance between the first layer and the hologram plane,and then the oblique distances from the other layers to hologram plane can be obtained from their simple relation,thus the computational time is much reduced.The CGH is optically reconstructed and the quality of reconstructed image is as good as that from the conventional algorithm.