Abstract: Based on the method of dimension analysis, the four-floor steel frame structure was designed as existing building(natural period 0.1~0.4 s) and the two-floor steel frame structure was selected as additional story to satisfy simulation ratio. According to the shaking table tests,the seismic behavior of extension-story structure, the isolation effectiveness and mechanism of isolation layer were compared and studied for the fixed base model, base sliding-isolation model, combined isolation model. The dynamic characteristic of prototypes were extrapolated by using finite element software. The results showed that:1)with no base isolation, the added layer would have dramatic increase of the dynamic response due to whiplash effect. 2)After adopting the isolation technology, the dynamic response of superstructures were greatly reduced. The decreasing amplitude ratio and economy of the base sliding isolation were better than the combined isolating system of foundation. Compared with the base sliding isolation, the inter-layer displacement amplitude of combined isolation had been reduced more drastically. 3)The original 4-story structure which the natural vibration period was 0.204 s with 2 added layers, the seismic response of structure was reduced and the seismic behavior of structure was improved after adopting the base isolation.