Abstract: The urban fringe of small and medium-sized cities exhibits pronounced spatial self-organization characteristics. Strengthening research into its self-organizing evolution can provide more scientific theoretical and methodological support for planning, policy formulation, and capital allocation. Using Xiaodongmen in Hengfeng County, Shangrao City, Jiangxi Province as a case study, this paper systematically analyzes self-organization phenomena across spatial morphology, street networks, architectural complexes, courtyard layouts, and micro-spatial configurations. By differentiating between inheritance and adaptation, urban and rural attributes, the following selforganization principles are identified: The motivation is the rising survival and development needs; The geographical scope is common in private and ambiguous property rights; The external energy is strong and the changes are in line with the local circumstances and the residents’ wishes are the conditions; The method is to form a center first,then spread through imitation, replication, improvement, etc. Finally, planning thoughts regarding intervention area, spatial texture, institutional mechanism, and community governance are put forward based on the interaction between self-organization and hetero-organization.