Abstract: The main objective of this paper is to propose a modified Perturb and Observe (P&O) maximum power point tracking (MPPT) algorithm for two-stage three-phase grid-connected photovoltaic (PV) systems. The previous-stage BOOST circuit achieves the maximum power point tracking control, and the rear-stage inverter realizes the stability of the DC bus voltage and the control of the grid-connected current. By adopting a novel variable-step size P&O algorithm, a fixed step size is adopted when the distance is far from the maximum power point, and the step size is reduced by half every time the working point crosses the peak point, until the final step size approaches zero and the operating point is stable at the maximum power point. The proposed algorithm effectively solves the mutual restriction between the tracking speed and accuracy of the conventional fixed step size P&O. The simulation results show that the variable step size P&O has better performance in tracking speed and accuracy than the fixed step size P&O. And the use of double closed-loop control at the grid side inverter can ensure the stability of the DC bus voltage and achieve unit power factor grid-connected control.