Aiming at the block-based fusion methods for multi-focus images, such key problems like finding a suitable block size and overcoming the blocking-artifact often occurs. To address them, we present an intelligent fusion method using particle swarm optimization (PSO) algorithm, which is guided by a structural similarity (SSIM) criterion. In this method, PSO is to fast search for an appropriate block size, while the SSIM is employed as the fitness function in PSO, in order to evaluate the blocking-artifact, thus enabling this method to have a significant improvement in fusion performance. Experimental results show that the proposed method performs better than some classical methods, such as a block-based method similar with ours in fusion rule and multiresolution-based method. Besides, our method preserves more details than others in fused image.