In this paper, a theoretical model for the analysis of effective-discharge parameters in a high-power transversely excited atmospheric pressure CO(2) laser with an electron beam preionization system is presented. The laser-discharge active volume and electron gun are modeled by nonlinear time-varying impedance. The lumped linear RLC circuits approximate the Marx generator and sustainer bank systems. By this method, the governing equations are reduced to a system of nonlinear time-varying ordinary differential equations that are solved numerically. Based on these equations, the electron beam and the main discharge parameters are evaluated. Our theoretical results are in good agreement with the experimental results.