To analyze the nonlinear mechanical behavior of concrete U-shaped thin-walled beams under pure torsion, a nonlinear analysis model was suggested based on Vlasov's elastic theory for open thin-walled beams, space truss model and numerical method of differential equation. Aiming at the character that Saint Venant torsion and warping torsion coexist in open thin-walled members, the nonlinear analysis models of the Saint Venant torsion stiffness and warping torsion stiffness of U-shaped cross-section were respectively derived, and nonlinear analysis model of open thin-walled members in pure torsion was derived. The nonlinear material constitutive laws of concrete and steel bars, the geometric nonlinearity of open thin-walled members, and the coupling effect of Saint Venant torsion and warping torsion were considered in the derived nonlinear analysis model. The corresponding analysis program was developed by using the derived nonlinear analysis mode under pure torsion. And 5 existing RC U-shaped thin-walled experimental members under pure torsion were analyzed with the derived nonlinear analysis model. Nonlinear analysis results correspond well with the experimental data. Analysis results confirm that the nonlinear analysis model derived here is correct. The derived nonlinear analysis model is simple and precise, the torsion nonlinear responses of concrete U-shaped thin-walled beam can be analyzed by the derived nonlinear analysis model, and it will provide a technical reference for the engineering design of such concrete members.