Passive hydraulic support units (PHSUs) are frequently used in the in-situ fabrication and testing (in-situ support). However, the difference in PHSUs’ stiffness will affect the mirror surface figure, especially for those thin meniscus mirrors. In order to solve this problem, the joint optimization method of layout, stiffness and active correction is studied. Firstly, for the difference of PHUS' stiffness, a hierarchical layout optimization method for support stiffness and support position is proposed to obtain the initial optimization solution of the support system. Then, the mode calibration method and the least square method is used for active correction of support system to obtain the final optimized solution of the mirror surface figure. Finally, the effectiveness of the method is verified by a numerical simulation experiment with specific cases. The results show that, for 4 m thin meniscus mirror, after layout optimization, with the hydraulic passive support system, the root mean square (RMS) of the mirror surface errors of 60 point axial support system is reduced from 150.6 nm to 32.9 nm, and the RMS value of the mirror surface errors of 78 point axial support system is reduced from 45.2 nm to 22.6 nm. The optimization effect is remarkable. After active correction, the RMS value of the mirror surface errors of 60 point axial support system is 14.6 nm, and it is 6.9 nm for 78 point axial support system. The requirement of the RMS value of the mirror surface error is less than λ/40 (λ=632.8 nm). The support systems meet the requirement. Finally, the 60 point axial support system was selected. Through the joint optimization of layout, stiffness and active correction for supporting points, it can greatly increase the applicability, flexibility and reduce the difficulty of implementation for the in-situ support system.