In the process of research and development of a new model, the retractor of load limited function of the rear seat belt cannot meet the requirements of the new version of C-NCAP. After establishing a simulation analysis model, by increasing the pretension function on the force limiting function of the rear seat belt, the simulation analysis found that there is a dummy diving problem, in order to prevent the dummy from diving, change the length of the lock webbing and other proposals to compare the simulation analysis, and finally confirm that the increase of the pretension function with the shortening of the lock webbing has better constraint performance on the neck and waist of the dummy. At the same time, it can reduce the risk of dummy diving, the neck Fz is significantly reduced, and the neck score is greatly improved, which provides a reference for the optimization strategy of the rear seat belt system and the optimization of C-NCAP score.