This research presents a new "intermirror flexible support structure" to mitigate surface shape errors in the mirrors of airborne bathymetric laser radar (B-LiDAR) optical systems under various working conditions. The flexible structure changes the mechanical relationship between the mirrors and support structure, thereby reducing mirror deformation. In this study, an optical system was first developed using Zemax and exported to Solid works to create a 3D model of the optical system. Subsequently, Ansys was used to analyze the face shape and conduct three different types of structural stiffness tests based on the actual working conditions. The analysis results reveal that the intrinsic frequency of the support structure using a ring-shaped thin cylinder is 331.06 Hz, peak-to-valley (PV) value of mirror deformation under thermal coupling is 37.25 nm, and root mean square (RMS) value is 14.52 nm. Moreover, dynamics analysis was performed for the support structure, and the maximum acceleration response of the mirror under the applied 1g acceleration excitation is 4.32g. Under random vibration, the maximum acceleration RMS value of the mirror assembly reaches 4.7g_rms and maximum stress of the flexible device of the support structure is 21.3 MPa, implying good mechanical characteristics. The maximum receiving field of view verification, optical system long-distance echo reception, and onboard bathymetry experiments were designed, which can effectively complete the echo reception. The results indicate that the flexible structure of the reflector support designed in this article is stable and reliable, and can be used in airborne depth measurement lidar systems.