The paper present a simplified new model for computing state equation and ionization degree of inert plasma, which is based on the Thomas-Feimi(TF)statistical model. We fitted the numerical results of the ionization potential calculated by TF statistical model and obtained the analytic function for computing the ionization potential. Then in the local thermal equilibrium (LTE) case we assume the density of plasma n(Z*) is a continuous function of Z*, and based on Saha equation with Debye-Hückel correction we established a simplified model of state equation of inert plasma. Using this simplified model we calculated the average degree of ionization and the state equation for Helium, Neon and Argon Plasma. The results from this simplified model are basically in agreement with both theory and experimental data. This simplified model can be used to calculate the state equation of plasma mixtures and is expected to have a wider applieation in the field of strongly ionized plasma.