Abstract: The major loss in the whiplash test of C-NCAP 2024 for a car seat assembly is in NIC and upper neck moment. Larger force on upper shoulder of dummy and insufficient stiffness of head restraint are the defects analyzed from the test result. The finite element model of seat is established, and optimization proposals are thus determined and proved by calculation. Seat sample is improved according to the proposals, and effectiveness of the scheme is verified through physical tests.