In order to achieve the mass spectrum at low vacuum, the equation of ion motion with resistance is derived. Pressure effects are treated by adding a drag term to the Mathieu function which is calculated based on fifth-order Runge-Kutta in MATLAB. The effects of gas pressure on stability regions and ion motion are described. A numerical method for getting stability regions is proposed. The first, the second and the higher stability regions in the presence of the damping force are given. The results show that the damping force caused by gas pressure enlarges the stability regions. The initially separate stability zones merge together for higher pressure. In low vacuum conditions, mass analysis can be performed by altering operation mode. The pressure effects can be reduced by increasing the frequency.