Abstract: ［Purpose/Significance］From the perspective of causal relationship, this paper explores the causal factors of mobile social media users' fear of missing out (FoMO), explores the theoretical model architecture and factors’ association path, and deeply analyzes the causal factors’ association of this phenomenon.［Design/Methodology］By using the grounded theory, both online and offline interviews were conducted to collect data. Open coding, spindle coding and selective coding were occupied to explore the causal factors of mobile social media users’ FoMO. Moreover, the NVivo 12 software and explanation structure model (ISM) of FoMO were made use of for an integrating analysis of the causal factors and their relationship, as well as the development of mobile social media users’ FoMO hierarchical structure model.［Findings/Conclusion］The results show that the FoMO formation mechanism model of social media users conforms to the S-S-O theoretical paradigm, and the hierarchical relationship model of mobile social media users' FoMO has been built. Besides, the functional relationship between the internal association path of elements at all levels has been discovered and analyzed in depth.［Originality/Value］This research is helpful to enrich the exploration of users’ mental health beyond the study of user information behavior. It is an academic explanation at the level of model architecture and factors’ correlation, which could provide theoretical and practical guidance for the study of social media users’ behavior.

Key words: Mobile social media, Fear of Missing Out(FoMO), Grounded theory, Stressor-Strain-Outcome Theory, Interpretative structure model(ISM)