The single coaxial pulse forming line is widely used in the field of high-power microwave technology. This paper focuses on the design of a single coaxial spiral pulse forming line with an output pulse width exceeding 30 ns. A field-circuit coupling numerical simulation model is established to investigate the effects of key structural parameters, including the inner-outer radius ratio \(\beta\) of the helix, the number of helical turns \(N\) , and helix spacing, on the output pulse waveform. Simulation results indicate that increasing \(\beta\) leads to a shorter pulse width and lower amplitude, attributed to changes in characteristic impedance and slow-wave factor. Increasing \(N\) widens the pulse width but reduces amplitude due to variations in the slow-wave factor. In contrast, helix spacing has a negligible effect on waveform characteristics, primarily impacting inter-turn insulation. These findings clarify the relationship between structural parameters and output performance, providing valuable guidance for the optimal design of spiral pulse forming lines.