The method of D-partitions is introduced to study the stability property of the reactor dynamic equation with six-delay-group representation. Two kinds of feedback models are considered—the delayed and the two-path temperature feedback. The stability region in parameter space, the effect of delayed neutrons, and the unconditional stability of the two-path temperature feedback are discussed in detail. The corresponding results of the one-delay-group model, one-group prompt-jump approximation, and effective lifetime model are also presented for comparing and discussing the validity or accuracy of these simplified kinetic models.