Objective To investigate if the inflammation mediated the association of body mass index(BMI) and the atherosclerosis. Methods A total of 4 508 subjects were randomly recruited from Gaoyou, and the indexes including the demographic characteristics, BMI, blood routine, biochemical test, disease history and brachial-ankle pulse wave velocity (baPWV) were measured. According to BMI, they were divided into low body weight group(BMI<18.5 kg/m²), normal weight group (18.5 kg/m²≤BMI<24 kg/m²), overweight group (24 kg/m²≤BMI<28 kg/m²) and obesity group (BMI≥28 kg/m²). The univariate and multivariate linear regression analysis was used to analyze the correlation between BMI and baPWV. The mediation analysis was used to evaluate whether white blood cell count was involved in the correlation between BMI and baPWV. Results After adjustment of age, gender, systolic blood pressure, fasting blood glucose, total cholesterol, low density lipoprotein cholesterol, alanine aminotransferase, aspartate aminotransferase, creatinine, smoking, drinking, diabetes and antihypertensive treatment, compared with normal weight group, the regression coefficient and 95% confidence interval of low body weight group, overweight, obesity group with baPWV were 1.01(0.51-1.51), -0.12(-0.27-0.02) and -0.28(-0.49- -0.08), respectively. The mediation analysis showed that white blood cell count mediated the correlation of BMI and baPWV, the mediation percentage of low body weight, overweight and obesity groups were 3.3%, 11.4% and 9.8%, respectively(all P<0.05). Conclusions The people of low weight could have a higher risk of atherosclerosis, and the inflammation might mediate the correlation between BMI and arterial stiffness.