Conspecific aggressiveness often increases after social isolation for species that are not entirely solitary, and this increased aggression could also be reversed after resocialization. However, literature on this aggression plasticity refers to either permanently social or low-level subsocial species in invertebrates. Examinations of conspecific aggressiveness reversibility in high-level subsocial invertebrates, in which offspring cohabitate with parents for a certain period of time after sexual maturation, would enhance the understanding of the role of conspecific-aggression plasticity in social evolution. Here, using the lactating spider Toxeus magnus, which exhibits extremely high-level subsociality, we assessed three questions. (1) Is its conspecific aggression affected by social living and/or kinship? The results indicated that conspecific aggression increased after social isolation, while kinship did not affect aggressiveness. (2) Could the social-isolation-induced higher aggression be reversed after resocialization? The results showed that the increased aggression of the spiders could be reversed 3 days after resocialization. (3) What is the proximate mechanism that caused the aggression reversibility by resocialization? A simulated resocialization experiment in which single spider was provided with mirrors demonstrated that the visual cues of conspecifics alone could reverse the aggression after 6 days. These results indicate that the high-level subsocial invertebrate showed aggressiveness reversibility without chemical cues. This is more similar to permanently social species rather than to low-level subsocial species, and visual cues could be vital to induce aggression change. These results suggest that conspecific-aggression reversibility might play a key role in social evolution and may functionally enhance species' adaptiveness under variable conditions.