Abstract: To research hypernormal form (simplest normal form, unique normal form) of nilpotent threedimensional vector fields, the problem of quintic hypernormal form of a class of nilpotent threedimensional vector fields with symmetrical property was studied by using new grading function and the method of multiple Lie brackets and by introducing the new marks for block matrices firstly, then that the second order normal form of above vector field was hypernormal form had been proved, and the unique form of the second order quintic hypernormal form was obtained.At last, the degenerate case of above vector fields was studied, and that this could be consistent with the conclusions in dimension 2 was verified.