Objective: To explore the longitudinal association between weight trajectories and physical fitness test composite scores among primary school students, so as to provide empirical evidence for school based weight management and physical health promotion. Methods: A total of 2 112 students from a primary school in Beijing who participated in physical fitness assessments annually from 2008 to 2023 were included and classified into different body weight trajectory groups based on body mass index (BMI) threshold: "normal/overweight-obese". Group based trajectory model was used to identify distinct weight trajectories. Generalized estimating equation and Cox regression were used to analyze the associations between weight trajectories and changes in physical fitness test scores, as well as event risks. Robustness checks were conducted. Results: The prevalence of overweight and obesity among primary school students was 26.50%. BMI showed a significant negative correlation with physical fitness composite scores ( r=-0.19, P <0.01). Four types of weight trajectories were identified: persistent normal group (64.5%, 1 362), persistent overweight- obese group (18.2%, 385), normal to overweight-obese group (14.0%, 296), and overweight-obese to normal group (3.3%, 69). Compared to the persistent normal group, both the normal to overweight-obese and persistent overweight-obese groups showed significantly higher risks of failing the physical fitness test ( HR =4.23, 4.60), and the speed of achieving excellent was slower ( HR = 0.52, 0.40) (all P <0.05). Robustness tests confirmed the stability of the findings. Conclusions Body weight trajectories have a long term impact on physical fitness performance among primary school students. Students with persistent or progressive overweight-obese trajectories have limited score improvement and a higher risk of failure; primary school students with persistent or progressive overweight and obesity should be a key focus.