The calculation speed is one of most dofficult problems for a computer-generated hologram of an object. This is due to the facts that both the pixel number of an object and that of the hologram are very large. In this paper we calculate the computer-generated hologram of an object by a difference method and translation symmetry. The main operation of the difference method is addition which is faster than multiplication and division. By translation symmetry of pixels of the object and holgram plane, we can void many repeating calculations. Thus oue method can speed up the computer-generated hologram of an objec. The computer simulation result proves that our method is effective.