Abstract: To meet the requirements of miniaturization of Marx generator and analyze the discharging process in field aspect, this paper introduces a new model of coaxial Marx generator and field-circuit co-simulation method. The coaxial configuration is based on ring component, avoiding field enhancement caused by structural eccentricity. Using field-circuit co-simulation function, charging and discharging process simulation is achieved by building structure and circuit model in CST software. Simulation results show the wave process in Marx generator. This method can offer help to insulation design and pulse analysis of Marx generator. The 5-stage Marx generator's volume is less than 0.015 m³, can be charged to 88 kV in 10 μs, the output pulse's peak voltage on 40 Ω load is 235 kV, peaking power is 1.4 GW, with 5.3 ns rise time, 11.2 ns pulse width.