Due to surface electromagnetic waves propagating along the dielectric-plasma interface, the application of surface-wave plasma (SWP) is limited in view of the fact that it is very difficult to realize metal sputtering by using negative-biased voltage in traditional SWP sources. Recently, this problem is overcome by a type of SWP source based on the guided wave in ion sheath layer driven by negative-biased voltage. And the plasma heating mechanism is originated from gas discharges excited by the local-enhanced electric field of surface plasmon polariton (SPP). However, the best discharge condition is not obtained because the influence factors affecting the discharge process studied is not clear. In this paper, the discharge mechanism of SWP ionization process influenced by gas pressure and negative-biased voltage along the ion sheath layer is investigated. The simulation method is by means of combining particle-in-cell (PIC) with Monte Carlo collision (MCC). Simulated results suggest that the values of negative-biased voltage and gas pressure can influence the thickness of ion sheath layer, the excitation of SPP, and the spatio-temporal conversion of wave mode, which further induces the different discharge performances. Moreover, the discussed analysis states that a better discharge performance can be obtained when approximately a negative-biased voltage of -200 V and a gas pressure of 40 Pa applied.