A DC-AC inverter system using a state observer and a control method thereof are provided. The control method of the DC-AC inverter system includes the following steps. The state observer outputs a filter-capacitor-current estimation value at a next sampling time according to a DC link voltage at a current sampling time and a filter-capacitor-voltage actual value at the current sampling time. The filter-inductor-current estimation value at the next sampling time is compared with the filter-capacitor-current estimation value at the next sampling time to obtain a load current estimation value at the next sampling time. An inductor voltage estimator outputs a filter-inductor-voltage estimation value at the next sampling time according to the load current estimation value at the next sampling time. A feed-forward control is performed according to the filter-inductor-voltage estimation value at the next sampling time.