The centrifugal model of medium-stiff free field is designed and constructed by using the laminar shear container. The dynamic centrifugal model tests of eight conditions are carried out in a 75g centrifugal field， and the artificial ground motion observation station is simulated. The results of acceleration peak， time history and response spectrum of the monitoring points with different depths in the medium-stiff free field are obtained. The ground motion amplification effects at different depths of the surface and underground of medium-stiff free field conditions are systematically studied. The results show that the medium-stiff field is weak of filtering effect. From the base to the surface， the amplitude and energy of the seismic wave gradually increase， and the seismic waveform loss is few. It is so obvious about the amplification of peak acceleration at the surface of medium-stiff free field. The ground motion amplification factor is between 1.86 and 2.49， and the amplification factor gradually increases from the base to the surface. The acceleration response spectrum of ground motion gradually moves to the high frequency direction from the base to the surface， and the response spectrum decreases with the increase of the free field depth. The ratio of the response spectrum of the ground and underground layers and the base seismic wave input is within a certain range， and there is an extreme point. The advantages are distributed between 1.65 and 1.85.