A set of hydrostatic support system for a 4 m SiC mirror in-situ testing was designed to reduce the risk of ultra-large mirror off-line testing and to improve fabrication efficiency. Firstly, the analytical formula for calculating support stiffness was derived, and its main factor was found. Then, some support samples were tested, and the support stiffness of a single support unit in different groups was predicted by combining the sample results and analytical formula. Finally, the stability of the support system was evaluated by pressurization test and in-situ testing, and the mirror surface precision with its gravity offloaded by the support system was calculated by finite element simulation. Results show that the average stiffness is about 1.9 kN/mm with a relative difference among support units about 3% when 5 units are linked together, the stiffness for a single unit isolated is as high as 15 kN/mm, and 3 kinds of unit stiffness in the support system are 1.7, 1.1 and 0.8 kN/mm respectively. Moreover, pressurization test shows a slow pressure change, which indicates that the system is well sealed. When the hydrostatic system was used for a 4 m mirror, the height changes rise up and down within 50 μm in 11 days, and the relative change is less than 20 μm. When the mirror is supported on 54 units with stiffness difference by 3% randomly, the surface RMS is less than 20 nm. The proposed system meets the requirement of in-situ optical testing for precision and stability.