A high-flying solar unmanned aircraft system capable of extending endurance time is disclosed. The system includes a main aircraft, a separable auxiliary power source and a connection device. The main aircraft includes a first body, a second body, a first wing portion, a second wing portion, a third wing portion, a first propeller and a second propeller. The second wing portion locates between the first body and the second body, and the second wing portion connects the first body and the second body. The connection device connects the main aircraft and the auxiliary power source, and includes a separation device. When the system climbs, the separable auxiliary power source provides additional energy to assist the main aircraft to climb. When reaching a preset altitude, the separation device, by burning out a line of connection bent, is turned on such that the auxiliary power source is separated from the main aircraft.