The assembly accuracy of thin-walled cylindrical parts in flight section of the aircraft is affected by multidimensional factors. As the initial source of assembly deviations, geometric errors directly influence the error transmission and accumulation characteristics of the assembly chain, serving as the foundation for systematic error modeling and regulation. This paper proposes an assembly error analysis method for thin-walled cylindrical parts based on small-displacement torsor theory. By mathematically modeling the geometric tolerances of key features of thin-walled cylindrical parts using smalldisplacement torsor theory and characterizing assembly deviations with homogeneous transformation theory, an assembly error propagation model for thin-walled cylindrical parts is established. The Monte Carlo method is employed to calculate and verify the qualification rate of assembly gradients through a combination of numerical computation and simulation analysis. The results show that the qualification rate obtained from simulation analysis differs by merely 1.25% from the theoretical calculation, validating effectiveness of the proposed model. By adjusting the tolerances of key assembly features, the qualification rate of thin-walled cylindrical parts assembly is improved from 90.90% to 99.90%. The theoretical method proposed in this study provides a reliable theoretical basis and practical reference for engineers in tolerance design.