BACKGROUND: The neurobiological mechanisms behind the antidepressant effect of bright light therapy (BLT) are unclear. We aimed to explore the dynamic functional connectivity (dFC) changes of the cingulate cortex (CC) in subthreshold depression (StD). METHODS: The StD participants (38 BLT and 39 placebo) underwent resting-state functional magnetic resonance imaging (rs-fMRI) and mood assessment before and after eight-week BLT. Seed-based whole-brain dFC analysis was conducted and multivariate regression model was adopted to predict Hamilton Depression Rating Scale (HDRS) and Centre for Epidemiologic Studies Depression Scale (CESD) scores changes after BLT. JuSpace toolbox was used to calculate the associations between dFC and neurotransmitter activity in the BLT group. RESULTS: BLT group showed decreased CESD and HDRS scores. Also, BLT group showed increased dFC of the right supracallosal anterior cingulate cortex (supACC)-right temporal pole (TP), left middle cingulate cortex (MCC)-right insula, and left supACC-pons, and decreased dFC of the right supACC- right middle frontal gyrus (MFG). Changes in dFC of the right supACC-right TP showed positive correlation with changes in CESD and HDRS. Moreover, combining the baseline dFC variability of the CC could predict HDRS changes in BLT. Finally, compared to baseline, the supACC and MCC dFC changes showed significant correlations with the neurotransmitter activities. CONCLUSIONS: BLT alleviates depressive symptoms and changes the CC dFC variability in StD, and pre-treatment dFC variability of the CC could be used as a biomarker for improved BLT treatment in StD. Furthermore, dFC changes with specific neurotransmitter systems after BLT may underline the antidepressant mechanisms of BLT.