Abstract: When nuclear heating reactor (NHR) is used as marine nuclear reactor, low rpm frequency conversion pump (FCP) is used in the intermediate circuit system (ICS). The vibration isolation system of FCP was studied in this paper. The effects of pipe stiffness on the natural frequencies of the system were studied through theoretical derivation and the ranges of vibration isolator parameters which ensure the pumps to operate normally were determined. Furthermore, the performance of isolation systems with various types and parameters was studied using finite element method. The design method of a qualified vibration isolation system for low rpm FCPs was presented. The results show that when the isolators are properly chosen for FCPs in the ICS, the natural frequencies of the isolation system are kept away from those of the pumps. Meanwhile, the isolation performance is satisfactory.