Based on the beginning, propagating and ending mechanism of rotating-stall cell, the relation between the pressure history signal and the pressure distribution along rotor circumference is proposed.The angular velocities of rotating-stall cell propagating are computed from time series picked by the pressure probes on a cross section.Self-relation calculating filtered the random noise of the pressure history data.The exciting load on rotor is computed by integral of filtered pressure signal along rotor circumference.By Prohl-Myklestad method, dynamical equations of rotor system are obtained.The dynamical response of rotor system is resolved by using Matlab system.Further more, the situation of more than one of stall cells is discussed.Two cases respectively from the natural gas compressor of some fertilizer plant and the CO2 compressor of some nitrogenous fertilizer plant demonstrate that both methods of calculating the load exerted on rotor by pressure fluctuation and resolving the dynamic response of rotor are available and the characteristics of frequency spectrum of rotating stall are correct.