Evidence on how different body composition indexes, such as body mass index (BMI), fat mass index (FMI) and fat free mass index (FFMI), influence blood pressure (BP) in Chinese children and adolescents is sparse. The aim of this study is to assess the effect of different body composition on BP in participants aged 7-18 in south China.A cross-sectional study was conducted in 2013. A total of 1,609 children and adolescents from urban and rural areas in Hainan Province, South China, were selected using multi-stage stratified cluster sampling method. Each participant underwent anthropometric and BP tests. BMI, FMI and FFMI were calculated and analyzed as body composition indexes.In the population level stratified by sex, FFMI had the highest r-value (Partial Spearman correlations) of correlation with systolic BP in boys (rs=0.305, P<0.001) and girls (rs=0.237, P<0.001). FMI was found to have the highest r-value of correlation with DBP in boys (rs=0.154, P<0.001). The quantile regression model indicated that, within each age group, all body composition indexes was positively associated with systolic BP, and FFMI has the greatest value of regression coefficient (4.38, 95% CI: 3.81-4.95). But for diastolic BP, positive associations with BMI, FMI and FFMI were only observed in the 11-12 years group. Restrict cubic spline models showed that there were both linear and non-linear relations between BMI, FMI and BP, but for FFMI, there was only linear association with BP.The relation between BP and FFMI was stronger than with BMI and FMI. Combined measurement of body composition should be considered to help health professionals interpret body mass data from children and adolescents with varied stature and age.