A vehicle seat assembly comprising a vehicle seat and an airbag assembly for this vehicle seat, wherein said vehicle seat comprises a seat base having two side surfaces, and a seat back having two side surfaces, said seat back being supported by the seat base, and said vehicle seat assembly comprises an air bag, mounted to the vehicle seat and capable of inflating from an undeployed state to a deployed state, wherein in the deployed state, the air bag extends along the head and torso of an occupant in the vehicle seat, so as to surround the head and torso of the occupant; characterized in that the airbag assembly further comprises a strap, for mounting the air bag to the vehicle seat and having a first section and a second section, wherein in the deployed state, the strap is tightened, the first section being mounted to the air bag on an outer surface of the air bag in an extension direction of the air bag, and the second section extending on at least one outer side surface of the air bag in a direction at an angle with respect to the extension direction of the air bag and the vehicle seat has diverting components for changing an extension direction of the strap, said diverting components being pulleys rotatably mounted on the vehicle seat, wherein the two side surfaces of the seat base are each provided with a first pulley; and the two side surfaces of the seat back are each provided with a second pulley, and two end portions of the strap are respectively connected to side surfaces of the air bag, and in the deployed state, on each side of the vehicle seat, the strap extends from the end portion of the strap sequentially around the second pulley and the first pulley. said vehicle seat comprises a seat base having two side surfaces, and a seat back having two side surfaces, said seat back being supported by the seat base, and said vehicle seat assembly comprises an air bag, mounted to the vehicle seat and capable of inflating from an undeployed state to a deployed state, wherein in the deployed state, the air bag extends along the head and torso of an occupant in the vehicle seat, so as to surround the head and torso of the occupant; characterized in that the airbag assembly further comprises a strap, for mounting the air bag to the vehicle seat and having a first section and a second section, wherein in the deployed state, the strap is tightened, the first section being mounted to the air bag on an outer surface of the air bag in an extension direction of the air bag, and the second section extending on at least one outer side surface of the air bag in a direction at an angle with respect to the extension direction of the air bag and the vehicle seat has diverting components for changing an extension direction of the strap, said diverting components being pulleys rotatably mounted on the vehicle seat, wherein the two side surfaces of the seat base are each provided with a first pulley; and the two side surfaces of the seat back are each provided with a second pulley, and two end portions of the strap are respectively connected to side surfaces of the air bag, and in the deployed state, on each side of the vehicle seat, the strap extends from the end portion of the strap sequentially around the second pulley and the first pulley.