Making use of the square-potential barrier to describe the influence of the insulating layer on the quasi-particle transport in normal metal/insulator/superconductor (NIS) tunnel junctions, we calculate the quasi-particle transport coeffic ients and the differential conductance using the Bogoliubov-de Gennes（BdG）equa tion and Blonder-Tinkham-Klapwijk(BTK) theory. It is found that the change of th e differential conductance with insulating layer thickness takes the form of the oscillation and attenuation, the period of oscillation and the speed of attenua tion depend strongly on the potential magnitude of the insulating layer and the bias value, the height of the conductance peaks and their positions are related to the insulating layer thickness intimately. It is shown that the tunneling spe ctra are different from those described by the δ-potential.