The Mid-story isolated (MSI) system is proved to be effective in reducing the seismic response of structures. Due to variability of the location of the isolation layer, modal coupling between higher modes may amplify the seismic response of MSI buildings, defined as modal coupling effect (MCE). In order to verify the results of theoretical analysis and evaluate the seismic response of MSI system, a series of shaking table tests were conducted with three scaled-down steel frame structural models with different frequency ratios between upper structure (US) and lower structure(LS). Three earthquake records with various peak ground acceleration (PGA) levels were selected as inputs of uniaxial shaking table tests. Results indicate that the acceleration responses of US of the case with MCE (denoted as Case 2) are apparently amplified compared with cases without MCE (denoted as Case 1 and 3).