To improve the fairness perception and norm enforcement of children and adolescents in inter-group interactions, it is an essential issue in the field of educational psychology about how to reveal the interactive mechanism of group norms and fairness values during individuals' psychological development. Although in-group favoritism during fairness norm enforcement is an important topic in psychology, three main limitations need to be addressed. First, most studies use questionnaires, which are easily affected by social participation. Second, most of the research uses scenario experiments with an emphasis on the final behavioral output of social interaction while failing to effectively grasp the dynamic process of social decision-making. Finally, electroencephalogram (EEG) studies with the high temporal resolution are still lacking to reveal the dynamic process of the brain. This project aims to clarify these issues by employing a cognitive neuroscience method. Specifically, multilevel techniques, including self-reported, cognitive-behavioral, eye-tracking, and electrophysiological techniques, were used to examine the mechanism behind in-group favoritism of fairness norm enforcement from childhood to adolescence. We intend to explore the developmental process, reveal the key role of cognitive control, mentalizing, and describe their psychological development trajectory. Findings will support moral education in primary and secondary schools, and cultivate students' sense of justice and behavior.