When occupants of different heights and weights are seated in the vehicle, the in-vehicle monitor will automatically determine the switching time of the second-stage load limiting. In the event of a vehicle collision, the ECU controller will first trigger the pretensioning device to detonate. At this time, the webbing will quickly retract inward, effectively pulling the occupant back toward the seat back for restraint. When the external load is too large, the webbing is pulled out, and the retractor then enters the locking state under the action of vehicle sensing and webbing sensing. The webbing pulls the reel to drive the main load-limiting rod to rotate, and at the same time, the main load-limiting rod will drive the secondary load-limiting rod to rotate. At this stage, a relatively high constant force acts on the occupant, which effectively controls the forward displacement of the occupant. However, considering that occupants of different heights and weights can withstand different forces, a secondary detonation will occur at this point, causing the second-stage torsion bar to disengage, leaving only the first-stage load-limiting rod in operation and entering the low-force stage. This effectively controls the pressure on the occupant's chest and waist, thus protecting the occupant's safety.