In order to alter the impedance feature of the base and the transmission properties from the excitation to the hull, an impedance changer represented by an M-K-C model is inserted between the lower isolator and the base of the raft system. In this paper, the influence of base impedance conversion on two single degree-of- freedom systems rested on an elastic base is studied. The result shows that the base impedance and the transmission properties from the excitation to the base have been changed, and the power transmitted into the base can be decreased at the resonant frequency of the hull. Similar conclusions can be made from the results of the raft system also. On this base, the influence of the impedance change of the base on the power transmitted to the base for complex raft system is studied. An optimization of the parameters of the impedance converser is made. This paper provides a theoretical reference for the design of the elastic base in raft vibration isolation systems.