Abstract: Disclosed are a side airbag system, and manufacturing and folding methods of the side airbag system, by which a passenger's head and thorax can be safely protected, and manufacturing cost and labor can be reduced. The side airbag system has an airbag cushion and an inflator assembly. The airbag cushion has a first panel, a pair of second panels, and pairs of reinforcement panels. The airbag cushion has an upper chamber and a lower chamber partitioned by a tether. The inflator assembly has an inflator, a mounting ring, and a mounting bracket. The tether constantly maintains a pressure in and a shape of the airbag cushion so that a gas injected by the inflator is filled in the lower chamber prior to the upper chamber.