Objective: To analyze the trajectory of dietary behaviors among middle school students in Jiading District, Shanghai, from 2021 to 2023, and longitudinally verify their association with anxiety and depression symptoms, aiming to provide scientific evidence for promoting the mental health of adolescents. Methods: The data were sourced from the National Monitoring and Intervention Project on Common Diseases and Health Impact Factors of students in Jiading District, Shanghai. A total of 1 217 middle school students who participated in at least two surveys from 2021 to 2023 were selected as the research objects, and group-based trajectory model was constructed to identify their dietary behavior scores trajectories. Modified Poisson regression was used to investigate the impact of dietary behavior scores trajectories on anxiety and depression, while Logistic regression was employed to explore the association between trajectories and changes in depression score levels. Results: The dietary behavior scores trajectories of middle school students were divided into three groups: Persistent Healthy Dietary Behavior (9.5%), Persistent Relatively Unhealthy Dietary Behavior (85.0%), and Persistent Very Unhealthy Dietary Behavior (5.5%). Students who perceived their academic performance as poor and whose parents had a cultural level of high school or below had a significantly lower proportion in the Persistent Healthy Dietary Behavior group compared to students with other characteristics ( χ 2=12.87, 8.69, 6.50, P <0.05). Compared with the Persistent Healthy Dietary Behavior group, the risk of anxiety symptoms in middle school students in the Persistent Very Unhealthy Dietary Behavior group was significantly increased ( aRR=3.04, 95%CI =1.15-8.02); Persistent Relatively Unhealthy Dietary Behavior and Persistent Very Unhealthy Dietary Behavior increased the risk of depressive symptoms ( aRR = 1.80 , 2.45, respectively), and were positively correlated with the increase in depression scores ( aOR =1.70, 2.24) ( P <0.05). Conclusions The dietary behavior of middle school students have not changed significantly in the past three years, with persistent unhealthy dietary behavior being the most common. Unhealthy dietary behaviors are positively correlated with the risk of anxiety and depressive symptoms and an increase in depression scores.