To understand different BMI suzhou the cognition and behavior of elementary school students to participate in extracurricular physical exercise, the research USES the literature material law, questionnaire survey and mathematical statistics research methods, such as the suzhou cognition and behaviour on the part of elementary school students to participate in extracurricular sports present situation investigation, analysis of existing problems and put forward the corresponding suggestion, in order to improve the suzhou elementary constitution to provide the reference. The results showed that the students with different BMI had better cognition of participating in extracurricular physical exercise, and the normal weight students had better cognition of the importance and liking of participating in extracurricular physical exercise than overweight and obese students. The extracurricular physical exercise behaviors of pupils with different BMI are diversified. The intensity, time and frequency of extracurricular physical exercise behaviors of pupils with normal weight are better than those of overweight and obese students. There is no significant difference in the cognition and behavior of extracurricular physical exercise among students with different BMI in Suzhou, and there is a certain correlation between extracurricular physical exercise index of students with different BMI.