The vibration isolation efficiency of spring vibration-isolation foundation is analyzed with single degree of freedom (SDOF) system, two-degree of freedom system and multi-degree of freedom system. The displacement theoretical formula of the isolation efficiency is deduced with two-degree of freedom vibration equation. It is concluded through vibration equation and finite element analysis that the general average isolation efficiency of the single degree of freedom system is on the high side, while the general average isolation efficiency of two-degree of freedom system is close to the finite element analysis result; the nominal unbalance phase has some effects on the calculated isolation efficiency and the square root of the sum of squares (SRSS）method can be used to calculate the isolation efficiency in actual engineering design.