To explore further, Modern telescopes are equipped with larger diameter mirror. Owing to deadweight and external force, it is difficult to maintain the position and attitude between primary mirror and mirror cell. Hydraulic supporting system is a optional choice to solve this problem, which requires high quality hydraulic oil and efficient oil filling. In this paper, the influence of gas content ( GC) on oil bulk modulus was discussed. Then an oil preconditioning and filling machine was designed. GC was detected using an oil particle size detecting instrument before and after preconditioning, and the result shows that GC decreases obviously from 0.1% to 0.002%. Oiling filling operation was also carried out on primary mirror test rig.