Researches on modeling method and dynamic characteristics of complex dual rotor system are carried out with a dual rotor experimental apparatus. The dual rotor system is divided into modal substructure and connecting substructure. Then, with finite element method and fixed interface modal synthesis method, modeling and dimensionality reduction of the modal substructure are conducted, and the connecting substructure is retained in the physical space. The substructures are assembled according to the butt conditions between supports and shaft. And then the dynamic equation of the dual rotor system with local nonlinear is solved using the Newmark algorithm. Validity of this method is verified by experimental results, and the rotor system response law is studied. The researches show that: with rotational speed changing, unbalanced response frequency and variable-stiffness vibration frequency of the rolling bearing and the combination of these frequencies emerge in the rotor response; law of unbalanced synchronous response has close relationship with supports’ position，unbalance of the rotor discs and the mode shapes of critical speeds.