Abstract: The effects of hydrogen injection pressure， ignition position and premixed ammonia concentration on the spark ignition characteristics of ammonia/air mixtures by high-pressure hydrogen jet were investigated using high-speed schlieren imaging and a constant volume combustion chamber （CVCC）. The results show that the high-pressure hydrogen jet velocity exerts a more significant influence on the initial flame kernel formation than the local fuel concentration. At the same injection pulse， there is no significant difference in unburned NH₃ emissions under different injection pressures. The ignition position should be carefully considered combining with hydrogen jet development. Lower injection pressures perform better with upstream ignition positions， while higher injection pressure may need mid-to-downstream positions to prevent local flow field disrupting the initial flame development. The radial ignition slows the flame spread due to leaner local mixtures but increases the flame area later， due to the extended ammonia-hydrogen mixing time. The approach reduces the unburned NH₃ emissions by over 20% compared to axial ignition. Richer premixed ammonia mixtures hinder the initial flame growth， while near-stoichiometric conditions （e.g. stoichiometric ratio of 1.0） speed up the flame expansion and increase the flame projected area. When stoichiometric ratio is 1.0， unburned NH₃ was reduced by 20% than the lean mixtures （e.g. stoichiometric ratio of 0.5）.