Abstract: The structure dynamic effect is a key factor to affect the stabilization accuracy. Currently, the most effective method to analyze the structure dynamics is to combine finite-element modal(FEM) analysis and experimental modal analysis. A two-axis and four-frame structure of a sight-stabilization system was taken as the research object of this paper. Firstly, the modal pre-analysis was completed using FEM. According to the vibration mode characteristics of simulation results, the modal for the out frames of two-axis and four-frame structure was tested. The natural frequencies, vibration modes and modal damp ratio were listed for the former 8-order modals. The mutual authentication between modal simulation and test shows that the accuracy level of the natural frequencies for two-axis and four-frame structure can be lower than 15% using FEM and the relevance of the vibration modes are good in the case of enough test point density.