The present invention provides a seat belt adjuster applicable to seat belts in vehicles, comprising: a pair of supporting swing rods composed of a first swing rod and a second swing rod, with the adjacent ends of the first and second swing rods hinged together and forming an included angle between the swing rods, either non-adjacent end of the first and second swing rods fixed in stationary state, and the other non-adjacent end of the first and second swing rods movable as the included angle between the swing rods changes; and an actuator, which changes the included angle between the swing rods by exerting force to the first swing rod and/or the second swing rod and/or the hinge point of the swing rods, and thereby directly applies variable force on the strap of the seat strap via the other non-adjacent end.