Background: The advancement of communication technology and the impact of the COVID-19 pandemic have led to an increased reliance on online education. However, the effects of the long-term use of smart devices for online learning on students? social anxiety and problematic smartphone use (PSU) and the role of fear of missing out (FoMO) in this process have yet to be fully explored. Methods: This study analysed longitudinal data from 2,356 high school students (female 5 1,137 (48.26%), mean age 5 13.84, SD age 5 1.37) in China, divided into high- and low-FoMO groups based on their scores on the FoMO scale, to examine the impact of four months of online learning on social anxiety and PSU. The Social Anxiety Scale (SAS) and Mobile Phone Addiction Index (MPAI) were used to assess social anxiety and PSU symptoms. Results: The undirected symptom networks revealed more bridge symptoms among the students in the high-FoMO group, although their overall symptom scores decreased. The results of the directed cross-lagged panel networks showed that ?productivity loss" predicted other symptoms in the low-FoMO group but that ?afraid of negative evaluation" was the predictor in the high-FoMO group. Meanwhile, ?withdrawal/escape" and ?productivity loss" were the symptoms that were most affected by other symptoms in the high-FoMO and low-FoMO groups, respectively. Conclusions: The current study therefore sheds light on the changes in social anxiety and PSU symptoms among secondary school students during long-term online learning, as well as the moderating role of FoMO.  Kulcsszavak: problematic smartphone use, social anxiety, fear of missing out, network analysis, longitudinal survey