Abstract: Twin spool rotor systems are widely used in modern aircraft engines. In order to reduce the weight of the rotorsystem, an inter-bearing is also used to support one end of the high pressure (HP) rotor between the high pressure(HP) rotor and the low pressure (LP) rotor in a twin spool rotor system. This structure can reduce the weight of the rotor system but make the coupling between the high pressure (HP) rotor and the low pressure (LP) rotor stronger. Thispaper sets up a simple model for simulating the vibration features of such a twin spool rotor. In the model the stiffnessand damping of supports and inter-bearing are taken into account. The stiffness of the inter-bearing has an obvious influence on the vibration of the twin spool rotor. When the stiffness is anisotropic, the gravity of the rotor system will resultin a vibration of twice HP speed, a "gravity critical"will appear. The maneuvering of airplanes will lead to an apparentincrease in the level of vibration excited by gravity. The LP rotor unbalance will cause a vibration with a combined frequency, and a resonance will take place. When the HP rotor runs opposite to the LP rotor, there is also one combinedfrequency. It turns out when an anisotropic stiffness of inter-bearing exists, more vibration components and more resonances will appear, this is detrimental to safe and reliable operation of a twin spool rotor.