A combustor of a water heater includes a burner member and a flame cover. The burner member has a passageway therein with an inlet and an outlet. The flame cover has a plurality of flame holes. The character of the present invention is that the burner member is provided with a room at a top thereof communicated with the passageway. A separating device is put in the room, and the flame cover is connected to the burner member to cover the separating device The separating device has a main outlet and two side outlets right under the flame holes of the flame cover to generate main flames and side flames respectively.