In the traditional optical system design process， optical designers focus more on optimizing the performance of the optical system without considering the as-built performance of these systems. To reduce the tolerance requirements for optical systems， this paper proposed a desensitization design method for freeform reflecting telescopes based on nodal aberration theory. This method provided mathematical expressions to describe aberrations generated by freeform terms on general decentered and tilted optical surfaces. To obtain the aberration coefficients of off-axis freeform surfaces， transformed pupil vectors were introduced into the aberration expansion， and only paraxial chief rays and marginal rays were traced to derive the aberration expressions. Then， an as-built performance evaluation model for the optical system was constructed based on this analysis framework. Using this method， two kinds of off-axis two-mirror and off-axis three-mirror systems containing freeform surfaces were designed， and the assembly sensitivity of the system was analyzed by 2 000 Monte Carlo ray tracing simulations. The results indicate that after about 10 minutes of optimization， the average wavefront aberration of the off-axis two-mirror telescope decreased by 26%， and the average wavefront aberration of the off-axis three-mirror telescope decreased by 14%， effectively verifying the effectiveness of the desensitization design method proposed in this paper.