Based on the original computer-generated hologram (CGH) of an object, a zooming algorithm of the reconstructed image of CGH is presented based on the Gerchberg-Saxton iterative algorithm. Based on the principle of object-image relationships and the setting of appropriate parameters, this algorithm achieves reproduction at any given zooming factor in the specified reproduction position, which can effectively avoid the problems such as uneven lighting caused by excessive enlargement zooming. Numerical simulation is operated on MATLAB platform a holographic reconstruction optical path based on liquid crystal on silicon spatial light modulator with pure phase is built to verify the algorithm. Reconstructed images of a binary picture with zoom factors of 0.5, 2 and 3 are obtained respectively at a specified diffraction position. A grayscale picture is enlarged to analyze the imaging effect of this algorithm. The experimental results show that the expected image zooming is obtained and the quality of reconstructed images is effectively improved.