Rust dismantling process using ultra-high pressure waterjet is carried out in a close vacuum cavity. When ultra-high pressure water flows out from nozzles, there is a temperature rise of 90°C generated. This temperature rise, combined with the powerful suction force produced by vacuum system and the atomization of waterjet brought by fast rotation of spraying rods, helps to achieve the process's goals, which are cleaning the metal surface to expose its "White Base", drying the cleaned surface just after dismantling and generating no rust in a determined period after treatment. In this paper, the numeric simulation for complicated flow field of multi-bundle ultra-high pressure striking waterjet in a restricted space is conducted with the help of Mixture model for air and fluid two-phase flow, the k-ω model for turbulence flow and the FLWEN+ software. The conclusions are: 1) the optimizing range for pressure and speed of flow field is S (the striking distance from nozzle outlet to cleaning surface) = 7 ～ 15mm; 2) the striking force and shearing force of high-speed waterjet vary with the changing of S. They reach their peak values when S = 15mm. This result is also consistent with the test results; and 3) the stability and convergence of k-ω model are the best among all applicable simulation models for turbulence flow, these lead to less computational time and higher efficiency.
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