BackgroundMindfulness-based interventions (MBIs) have been demonstrated to enhance emotional functioning, but little is known regarding the brief mindfulness training’s effects on various psychological symptoms and positive psychological resources in the long term, especially when compared to both passive and active control conditions. This study aimed to examine the effects of brief mindfulness training on psychological symptoms compared with active and passive control conditions across three time points.MethodsThis study was designed as three-wave longitudinal, three-arm randomized controlled trial. (N = 180) participants were randomly grouped into mindfulness training group (MG), breathing relaxation group (active control, ACG), and no-treatment control group (passive control, PCG). The psychological outcomes (anxiety, depression, state and trait mindfulness, attentional control, emotional intelligence, and psychological resilience) were determined at baseline (T1), post-intervention (T2), and 2 weeks follow-up (T3). Psychological outcomes were assessed using validated self-report instruments, including the Self-Rating Anxiety Scale (SAS), Self-Rating Depression Scale (SDS), State Mindfulness Scale (SMS), Five Facet Mindfulness Questionnaire–Short Form (FFMQ-SF), Attentional Control Scale (ACS), Emotional Intelligence Scale (EIS), and Connor–Davidson Resilience Scale (CD-RISC). Repeated-measures ANOVAs were used to test Group × Time effects.ResultsThere were no significant differences in baseline between groups. There was a significant Group × Time interaction in all the measured outcomes. The mindfulness group (MG) compared to both control groups had greater post-intervention anxiety and depression reductions and state and trait mindfulness, attentional control, emotional intelligence, and resilience increases. The ACG showed slight within-group changes but not better than the PCG.ConclusionBrief mindfulness training had specific, time-varying changes in psychological symptoms and positive psychological resources. These results suggest that mindfulness has more than a general relaxation or engagement effect, which demonstrates its effectiveness as an intervention that can redefine the psychological functioning by using dynamic, targeted attentional regulation processes.