In this paper, the influence of thrust and air drag on rocket lateral vibration is studied, and the frequency reduction under the thrust and drag is analyzed. The formulas for evaluation of the changes of frequencies and vibration modes are obtained. The equations of transverse movement when nozzle connected rigidly with rocket and related boundary conditions form the basis for analyses. The conclusions obtained are applicable for active accelerating and passive decelerating parts of trajectory as well as the cruise flight when thrust and drag are balanced. The analyses show that for small thrust and small axial overload, the discrepancy resulted from different calculation methods is not big, whereas for the heavy overloaded case a more accurate solution is extremely necessary.