Abstract: Based on analogy theory,1:4 water model has been established for 300 t RH refining unit at Masteel The effects of different immersion depth on circulation flow, mixing time and residence time of vacuum chamber were investigated. Test results show that the circulating flow rate increases with the increase of immersion depth. When the immersion depth is greater than 500 mm, the upward trend of circulating flow slows down. When immersion depth is greater than 500 mm,the mixing time is low. When the immersion depth reaches 520 mm,the rising trend of the residence time of the vacuum chamber begins to weaken. When the immersion depth exceeds 560 mm,the residence time of the vacuum chamber changes little. Taking account of the above factors, the optimum immersion depth of 300 t RH at Masteel should be controlled within 520 ~ 560 mm.