Abstract: ObjectiveTo study setup errors correction through the image guided real-time online in intensity-modulated radiotherapy,calculate external radiation therapy the margin ranges,and analyze weight difference influence on setup in cervical cancer patients. Methods Cervical cancer patients treated by Intensity-modulated radiotherapy,were divided into group A (BMI≥24) and group B (BMI＜24)according to the body mass index.Obtained CBCT and planning CT images were automatically registered in gray scale,and setup errors were calculated and evaluated online.. Results Setup errors of 37 patients in the X axis (left-right),the Y axis (head-foot) and Z axis (superior-inferior) direction were (1.1±2.3),(2.1±5.0) and (1.1±2.2) mm respectively,and the margins in X,Y and Z axis were 5.2mm,11.0mm and 5.6 mm respectively.Setup errors in group A in the Y axis direction was larger than those in group B,P＜0.05.And the differences in both X axis and Z axis were not statistically significant between two groups (P＞0.05). Conclusion CBCT online correction could reduce the cervical cancer patients setup errors,improve the accuracy of radiotherapy and estimate the margins.In patients with increased Body mass index,radiotherapy margins should be appropriately expanded.