The dynamic characteristics of gas lubrication bearings system was studied on the dualturbo expander generator test bed with the fourdisk and a radial turbine rotor structure supported by hydrostaticdynamic hybrid gas lubrication bearings.Based on shaft center orbits,frequency coupling 3D plot and frequency spectrum,the nonlinear characteristics of rotor bearing system were analyzed.The reason of a sharp increase in rotor speed was analyzed combined with the structure of the rotor system.The experimental results prove that,because of the application of gasbearings supporting structure in the bearingrotor system,gas film halfspeed whirl and gas whip appear under the certain bearing supply pressure,and the speedrise appear under the certain speed.Due to low damping and rotor structure，a sharp increase in rotor speed appears when the energy of power frequency and fractional frequency transform into rotor speed.The measures such as adjusting the vibration characteristics and controlling the flow of gas turbines can suppress the speedrise.