Abstract: Topological insulator is one of the most attractive areas in condensed matter physics in recent years.The study of the dephasing effect in such systems has important theoretical meanings and meanwhile,is inevitable for the realization of future quantum devices.In our studies of the influence of dephasing on the two-dimensional topological insulator,especially the quantum spin Hall effect，we find that the quantized longitudinal resistance plateaus are robust against normal dephasing but fragile to spin dephasing. Thus, these quantized plateaus can only survive in mesoscopic samples in which the spin coherence length is smaller than the sample size. Our results are in excellent agreement with experiments (Science,2007,318:766). In addition, we define a new spin Hall resistance that also exhibits quantum plateaus, which are robust against both normal and spin dephasings. These novel quantum plateaus can thus survive in macroscopic samples and better reflect the topological nature of the quantum spin Hall effect.