To meet the demanding requirements of the aeronautical optoelectronic platform for surface accuracy and dynamic performance of the visible/infrared dual-band two-axis fast steering mirror， a lightweight flat mirror assembly and a flexible structure have been specifically designed. First， the design method of the fast steering mirror structure is summarized， their design elements are clarified， and the influence of assembly error is analyzed. A lightweight mirror supporting the center of the back is designed， and the high-precision assembly of the motor is realized through a positioning fixture. Then， based on the cross-shaped flexible bearing， the design of the biaxial flexible structure is realized. Finally， the surface accuracy and mode of the fast steering mirror are simulated and tested. The test results show that the surface shape accuracy （RMS） of the flat mirror is greater than 0.017λ （λ=632.8 nm）， the closed-loop bandwidth of the fast steering mirror is higher than 200 Hz， and the position errors in both the X- and Y-directions are less than 1.2 μrad. As a result of achieving high surface accuracy and dynamic performance， the miniaturization and modularization of a fast steering mirror are realized， allowing them to function stably and reliably in complex aviation environments.