In this paper, We first fitted the numerical results of the ionization potential calculated by Thomas-Fermi statistical model and gained the analytical function of the potential versus the degree of ionization, then calculated the ionization potential and the average degree of ionization for argon versus temperature and density in local thermal equilibrium (LTE) case. The curve of pressure versus specific volume V of argon plasmas is calculated according this simplified model. The calculated results by this simplified model are in agreement with experimental data and the calculated results of the Saha-equation theory. This simplified model can be used to the calculation of the equation of state of plasmas mixture and is expected to be widely used in the field of EML technology involving the strongly ionized plasmas.