Image captioning takes image as input and outputs a text sequence. Nowadays, most images included in image captioning datasets are captured from daily life of internet users. Captions of these images are consequently composed of a few common words and many rare words. Most existing studies focus on improving performance of captioning in the whole dataset, regardless of captioning performance among rare words. To solve this problem, we introduce long-tail image captioning with dynamic semantic memory network (DSMN). Long-tail image captioning requires model improving performance of rare words generation, while maintaining good performance of common words generation. DSMN model dynamically mining the global semantic relationship between rare words and common words, enabling knowledge transfer from common words to rare words. Result shows DSMN improves performance of semantic representation of rare words by collaborating global words semantic relation and local semantic information of the input picture and generated words. For better evaluation on long-tail image captioning, we organized a task-specified test split Few-COCO from original MS COCO Captioning dataset. By conducting quantitative and qualitative experiments, the rare words description precision of DSMN model on Few-COCO dataset is 0. 602 8%, the recall is 0. 323 4%, and the F-1 value is 0. 356 7%, showing significant improvement compared with baseline methods. © 2022 Beijing University of Aeronautics and Astronautics (BUAA). All rights reserved.