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Purpose To investigate the prevalence and risk factors of overweight and obesity among children and adolescents in Hangzhou, thereby informing the development of tailored preventive strategies.Methods This school-based cross-sectional study conducted population-level analysis on 353 783 individuals across four districts. Using stratified cluster random sampling, we surveyed 6148 students from 28 schools through self-administered structured questionnaires to collect data on influencing factors. Univariate and multivariable logistic regression were used to screen possible risk factors of overweight and obesity.Results The overall overweight and obesity rates were 27.20%, showing a significant decline in obesity prevalence with age (χ 2=84.25, p<0.001), in contrast to stable overweight rates. Boys had consistently higher rates of overweight and obesity than girls of the same age (χ2=69.79, p<0.001). The prevalence of overweight and obesity in urban students was higher than that in suburban students (χ2=2.01, p=0.023). While overweight prevalence generally exceeded obesity prevalence across age groups, this pattern reversed among children aged 6–10 years, where obesity prevalence equalled or exceeded overweight prevalence, particularly among boys. After puberty, eating fresh fruit ≥1 time a day, never eating breakfast, brushing teeth ≥2 times daily and bullying victimisation were independently associated with a higher prevalence of overweight and obesity. Moreover, female sex and boarding student status were protective factors against overweight and obesity (all p<0.05).Conclusions In conclusion, before or after puberty, gender, boarding at school, higher fruit consumption frequency, breakfast skipping, increased toothbrushing frequency and bullying victimisation were significantly associated with overweight and obesity in Chinese children and adolescents. It is recommended that schools, families, hospitals and society should jointly take measures to encourage adolescent students to cultivate good learning-related behaviours and healthy lifestyles to maintain and promote physical health.