Multi-focus image fusion is an increasingly important component in image fusion, and it plays a key role in imaging. In this paper, we put forward a novel multi-focus image fusion method which employs fractional-order derivative and intuitionistic fuzzy sets. The original image is decomposed into a base layer and a detail layer. Furthermore, a new fractional-order spatial frequency is built to reflect the clarity of the image. The fractional-order spatial frequency is used as a rule for detail layers fusion, and intuitionistic fuzzy sets are introduced to fuse base layers. Experimental results demonstrate that the proposed fusion method outperforms the state-of-the-art methods for multi-focus image fusion.