Single Decker Ball Bearing (SDBB) is widely used in Rotor-Bearing system. A new method using DDBB composed of two ball bearings as support bearings is proposed. The mechanical model of the DDBB based on the quasi-dynamic method is established and the corresponding calculating program compiled in Matlab is developed after considering the radial load, axial load, centrifugal force as well as gyroscopic moment acted on the bearing simultaneously. And then a simple Rotor-DDBB model is adopted to analyze the rotor unbalance response with different parameters. The simulation results show that shaft rotating speed, ball materials, axial preload and the initial contact angles to some extent impact the bearing stiffness while have little affects on system nature frequency and the rotor unbalance response which greatly affected by the system base stiffness. The results provide a theoretical basis for the design of DDBB and application in a Rotor-Bearing system.