In this paper, we reviewed the symbiotic association between animals and nitrogen-fixing bacteria and the nature and mechanism of symbiosis. A total of 174 species of animals in 7 phyla, 13 classes, 23 orders, 50 families, and 99 genera were evaluated in this paper, and they were found to use acetylene reduction and gene detection of nitrogenase. The animal gut has a rich microbial environments, which meets the growth and development demands of nitrogen-fixing bacteria. As a result, the animal guts were rich with symbiotic nitrogen-fixing bacteria of different ecological types, such as those that are symbiotic with plants, those that are endophytic diazotrophs, nitrogen-fixing bacteria from the rhizospheres, and Azotobacter. It is generally believed that animal symbiosis with nitrogen-fixing bacteria stems from the environment. The nature of animal symbiosis with nitrogen-fixing bacteria belongs to associative nitrogen fixation. Nitrogen-fixing bacteria that are symbiotic with animals coordinate with other symbiotic organisms to meet the needs of electron and proton donors, energy supply, nitrogenase activity protection, and ammonia repression relief during nitrogen fixation. The assimilation of ammonia during nitrogen fixation is also involved in the synergies of different symbiotic organisms and might be carried out through the pathway of glutamine synthetase/glutamate synthetase.In general, food nitrogen, non-protein nitrogen, and nitrogen-fixing symbiotic bacteria coordinate with each other to form a metabolic network of nutrition and detoxification to maintain the homeostasis of nitrogen nutrition in animals. The prospects of nitrogen fixation of animal symbionts were also identified in this paper.