The phase characteristics of stiffness increase phenomena of a rubbing rotor are investigated. The full annular contact may be maintained for a wide speed range even though the rotor has passed through its critical speed. It is verified that for stationary rub, the phase has an inherent characteristics, that is, the phase difference between the response and the excitation approaches to gradually along with the increased rotor speed; the phase difference will jump to after it exceeds with a small increment, and then non-rub motion start. The analytically and numerical results give mechanism of stiffness increase. The phase characteristics of non-stationary rub motions after Hopf bifurcation have also be numerically analyzed.