Background. Burnout is a common psychological syndrome among nurses, especially in the first few years of working. However, limited studies have evaluated the factors related to burnout among junior nurses. Aims. To investigate the levels of psychological flexibility, stress, and burnout among junior nurses and examine the role of demographic and workplace relationship factors, psychological flexibility, and perceived stress on burnout among junior nurses. Materials and Methods. A convenience sample of 481 junior nurses was recruited from three tertiary hospitals in China from July 2021 to August 2022. Data were collected, including demographic data, workplace relationships, psychological flexibility (measured by experiential avoidance and cognitive fusion), perceived stress, and burnout. The Independent t-test, one-way ANOVA, Wilcoxon–Mann–Whitney test, Kruskal–Wallis test, Spearman correlation, and hierarchical regression were used to analyze the data. Results. Junior nurses had a high level of burnout in different dimensions. Nurses with an undergraduate education, nurses working in the first year, and a lack of support from supervisors and poor colleague (nurse-nurse and nurse-doctor) relationships had the lowest level of psychological flexibility and the highest level of perceived stress and burnout. Spearman correlation indicated that experiential avoidance, cognitive fusion, perceived stress, and burnout were positively associated with each other. The regression model showed that psychological flexibility (especially cognitive fusion) and perceived stress influenced burnout in junior nurses. Conclusions. Higher psychological flexibility and lower perceived stress appear to improve burnout in junior nurses. Therefore, interventions targeting these two factors may provide a viable direction for the reduction of burnout among junior nurses. Implications for Nursing Management. Hospital authorities should create a harmonious working environment and provide some psychology training programs for junior nurses.