To improve the stability and dynamic performance of the inverter in a weak grid system, a multi-complex coefficient filter (MCCF) based grid impedance estimation and voltage feedforward method is proposed for a three-phase inverter. The grid impedance is estimated by injecting a small harmonic current and measuring the response using MCCF. The real grid voltage can be calculated by considering the voltage drop across the grid impedance. Compared with the conventional feedforward control using point common coupling (PCC) voltage, the feedforward of real grid voltage can eliminate the positive feedback caused by the grid impedance. Adaptive adjustment of current loop parameters can also be achieved according to the grid impedance. The results show that real grid voltage feedforward can improve the phase margin and the stability of the inverter system.