This paper describes the connotations and elements of user fear of missing out (FoMO) in mobile micro-media environment. The possible mental state of FoMO users is analyzed based on S-O-R model and cognitive psychology. The mobile micro-media user FoMO generation mechanism is consised by 7 sub-mechanisms such as induction of mild incentives sub-mechanism, shifting of attention sub-mechanism, triggering of empathy mechanism sub-mechanism, need of cognitive connection sub-mechanism, adjustment of cognitive dissonance sub-mechanism, drive of perceptual control sub-mechanism and loss of mindfulness perception sub-mechanism. Finally, the authors explain relationship between elements and sub-mechanisms and analyze logical relationship among sub-mechanisms. This paper interprets the internal psychological changes and external irrational, non-adaptive and problematic media use behavior of FoMO users. It will sever as an explanation and reasoning reason, which will help users to defend, regulate and control negative emotions such as anxiety, suffering and loss. The paper extends the research on user psychology and information behavior.