The ground current for grid connected transformerless photovoltaic inverter system is analyzed. For the single-stage structure， the traditional three-phase inverter and the three-phase half-bridge inverter with split capacitor are discussed. For the two-stage structure， the two-level boost converter plus three-phase inverter and the three-level boost converter plus three-phase inverter are discussed. The calculation method to estimate the ground current is established. The common-mode models and the Fourier equations of the ground current are given. And the effect of different parameters on the ground current is proposed. The influence of different front-end boost converters on the ground current is analyzed. The simulation and experiment results are given to verify it.