Ammonia has emerged as a viable hydrogen energy carrier owing to its superior hydrogen density and mature industrial utilization. However, ammonia faces critical challenges including inadequate ignition characteristics and sluggish combustion kinetics, necessitating supplementary high-reactivity fuels for optimizing combustion. Onboard ammonia decomposition technology resolves this problem through on-demand hydrogen real-time production. Among existing ammonia decomposition methods, gliding arc plasma (GAP) demonstrates exceptional promise for onboard hydrogen production given its high processing flow rate，decent hydrogen conversion rate, and transient response capability. Prevailing research predominantly relies on experimental approaches, with insufficient understanding of the effects of specific electrical field parameters and inlet pressure on system performance. This study established a quasi-one-dimensional numerical model for GAP-assisted ammonia decomposition. A comprehensive analysis was conducted to examine the influence of key electric field parameters, such as reduced electric field strength (REFS) and electron density (De), on ammonia conversion rate and energy efficiency. Furthermore, the study explored the synergistic effects of inlet pressure and electric field parameters on system performance under constant mass flow rate conditions. The results indicate that increasing REFS and De significantly substantially elevates ammonia conversion rate, but energy efficiency decreases as these parameters increase. Keeping a constant NH3 inlet mass flow rate, the gas velocity decreases when the inlet pressure increases and then extends the residence time. Consequently, the ammonia conversion rate significantly improves while the energy efficiency slightly decreases. By increasing inlet pressure and simultaneously reducing REFS or De, system energy efficiency can be effectively enhanced without altering ammonia conversion rates. This study demonstrates the synergistic regulation mechanism of electric field parameters and inlet pressure on hydrogen production performance, providing optimization strategies for GAP reactor design.