The circuit analysis of SPD and parasitic inductance in the connecting wire were done.It was pointed out the influence of parasitic inductance in the wire on SPD installment.The parasitic inductance could not only weaken the clamp limiting voltage function of the protective device,but also realized the energy coordination between the multistage SPD by using the parasitic inductance reasonably.And it was expounded,through specific example,we should know the parasitic inductance scientifically in order to reduce the harm of parasitic inductance to devices which had been protected.