A surface-wave plasma (SWP) source based on surface plasmon polariton (SPP) has fine performances such as high density, low temperature, high production, and so on. It is applied in electronic device micro or nano processing, material modification, etc. Because the ionization of SWP heated by SPP is difficult to describe by theoretical analysis and experimental measurement, the ionization process of producing uniform stable SWP source is not yet well understood. The method in this paper is a numerical simulation of SWP discharges. The electromagnetic energy coupling mechanism of ionization process, influenced by gas pressure, which is studied by combining particle-in-cell (PIC) simulation of reciprocity between plasma and electromagnetic wave with Monte Carlo Collide (MCC) method in merit of dealing with particle collision. Simulated results suggest that the efficient production of SWP is induced by locally enhanced electric field of SPP, and the gas pressure influences the ionization process of SWP by altering the appearance time of wave-mode resonant conversion. Results of this paper show the ionization process of SWP discharge maintained by SPP, and further provide some advices for designing the parameter optimization of next generation meter-scale SWP source.