The rotary draw bending of thin-walled rectangular tube is a complex process with multi-nonlinearity under multi-die constraints. The constraints of various dies on the cross-section distortion ratio are studied by experiment and the friction coefficients are obtained by twist-compression test. It is found that the effects of bend die, mandrel die and pressure die on cross-section distortion are significant. The significant influencing factors include bend angle, bend radius, clamp pressure, core number and the boost velocity. A three-dimensional finite-elements model of this process is built under the ABAQUS/explicit environment based on the solution of several key techniques, such as model creation, material properties definition, contact boundary condition treating, meshing technology, and it is validated to be credible. Furthermore, the orthogonal experimental design is used to investigate the interactive effects of friction on cross-section distortion ratio, and the optimal combination is obtained.