An analytical expression and a perturbation formula for the cavity resonant frequency dependence upon plasma are derived for the cylindrical TM 010 microwave plasma cavity. A comparison between the exact and the approximate solutions showed that the perturbation theory can be used accurately to analyze the perturbating effect on the cavity resonant frequency caused by microwave plasma in low pressure microwave excited gas lasers. A new method is proposed to prevent the detuning of the cavity from the perturbation of the plasma and the glass tube, so that a uniform and steady-state microwave plasma is successfully formed and the stable laser output is also obtained. The electron density and microwave dielectric constant of the tube material can be measured by the perturbation approach.