Many unbalance related vibration phenomena can not be reasonably explained by rotor isotropic hypothesis theory. Based on the characteristics of anisotropic of stiffness and damping in rotor system, the dynamic model of the nonlinear stiffness rotor system was set up. The changing law of rotor’s unbalance response was researched on the conditions of ignoring and considering about nonlinear stiffness coefficients respectively. The results show that: for the linear modal, the trajectory of unbalance responses of different imbalance is a set of concentric ellipses. The initial phase angles of those ellipses go along with changing angle of unbalance; for the nonlinear modal, the azimuth angle and the eccentricity of unbalance response due to different imbalance may change, and show strong nonlinearity. This result provides theoretical basis for the application of full vector dynamic balancing.