The airbag system for vehicles includes an airbag housing, a sliding door and a connection member. The airbag housing has a gas injection hole, through which inflation gas is injected into an airbag cushion when a vehicle collision occurs. The sliding door moves over the gas injection hole to control the amount of inflation gas injected into the airbag cushion. The connection member is connected between the airbag cushion and the sliding door such that the sliding door is moved by the inflation of the airbag cushion.