Background: Previous studies have indicated that disease activity, psychological symptoms and social support were associated with health-related quality of life (HRQoL) in patients with inflammatory bowel diseases(IBD). However, it is unclear how disease activity, psychological symptoms and social support interact to affect HRQoL. The main purpose of this study was to examine the mediation effect of psychological symptoms in the relationship between disease activity, social support and HRQoL. Methods: This was a cross-sectional study, which collected data using convenience sampling, between December 2016 and March 2018, from the Third XiangyaHospital of Central South University in Changsha, China. An online self-administered questionnaire (including demographic and clinical information), Inflammatory Bowel Disease Questionnaire, Disease Activity Indices scale, Hospital Anxiety and Depression Scale and Social Support Rating Scale, were administered to each participant. Descriptive statistics and Pearson's correlations were used to summarize data, whereas PROCESS analysis was performed to examine the pre-specified mediation effect. Results: A total of 199 patients with IBD were included. Disease activity indices (DAI) and hospital anxiety and depression (HAD) were negatively correlated with HRQoL (β = − 3.37, − 2.54 respectively, P < 0.001), while social support was positively correlated with HRQoL (β = 1.38, P < 0.01). HAD partially mediated the negative relationship between DAI and HRQoL (β = − 0.83, P < 0.001) with the mediation effect ratio of 24.6%, and completely mediated the positive relationship between social support and HRQoL (β = 1.20, P < 0.001). Conclusions: Psychological symptoms acted as a mediator in the relationship between disease activity, social support and HRQoL. Interventions to improve HRQoL in patients with IBD should take into account the mediation effect of psychological symptoms.