Objective To investigate the distribution and the influencing factors of body mass indes(BMI) in primary and middle school students in Lanzhou in 2014. Methods A total of 4 200 students aged 7～18 in Lanzhou were selected by stratified random sampling. The height,weight of students were measured to calculate BMI. The questionnaires were from the detailed measurement rules about Investigation of the Chinese Students′ Physical Fitness in Year 2010. χ² test and multivariate-Logistic regression analysis were used to analyze influencing factors. Results Totally 13.02%,73.52%,9.36% and 4.10% of the 4 200 students were with underweight,normal weight,overweight and obesity respectively. There were significant differences on BMI in children with different genders and dwelling place(P＜0.05). Multivariate-Logistic regression analysis showed students′ obesity and overweight were related to district,sex,sleeping-time,frequency of breakfast (OR=1.67,2.07,1.52,1.39,all P<0.05).And underweight in students was associated with district,sex,sleeping-time,frequency of having milk,watching TV(OR=0.71,1.31,1.56,1.40,0.81,all P<0.05). Conclusions The distribution of BMI and obesity status vary with gender,district and ages. Also,the health of primary and middle school students can be improved by strengthening health education and developing healthy diet and living habits.