For analysis on pull-rod bent focusing mirror assemblies in engineering, the accurate results can be obtained by a contact nonlinear method rather than a linear analysis. As analysis of a nonlinear system by linear method shows a bigger error, this paper researches the causes of errors. A pull-rod bent focusing mirror assembly is simulated by the finite element method, then the contact nonlinear analysis method is introduced to analyze the response of focusing mirror when its pull-rod is bended. In comparison of the nonlinear analysis results with detection results, it is pointed out that the errors between them on scope and radius are 4 μrad and 5 km when the bent vector height of focusing mirror is a constant, which can meet the requirements of engineering. Finally, the tested results of the bent focusing mirror from linear analysis, nonlinear analysis and detection are compared, which indicates that the nonlinear analysis is closer to a real system in engineering analysis and suits to simulate the real conditions at the scope of limited errors.