To understand the effect of dies on the cross-sectional deformation of thin-walled rectangular H96 brass tube during the small-radius rotary draw bending (RDB) process, the reliable 3D FE models with eight different die sets were established based on ABAQUS platform, and the cross-sectional deformation of thin-walled rectangular tube with different die sets was investigated. The results show that the mandrel can obviously suppress the cross-sectional deformation of thin-walled rectangular tube. The wiper die can improve the cross-sectional deformation under the die set of base die set+mandrel+booster die, but it will increase the deformation when other die sets are used. Adding a booster die can improve the cross-sectional deformation, but when the die set is different, the degree of improvement is also different. The effect of each die on cross-sectional deformation of rectangular tube during small-radius bending is clarified, and optimal die set can be reasonably selected according to the requirement of cross-sectional deformation in engineering applications.