Jet orifice diameter directly impacts the combustion process of the pre-chamber jet ignition (PJI) engine and the optimized diameter is varied with the fuel properties. However, research on the optimization of the jet orifice diameter based on aviation kerosene fuel has not been reported. So, this paper investigates the effect of orifice diameter on combustion, pressure oscillation, and performance based on a kerosene-fueled single-cylinder test engine. Two pressure sensors are respectively fitted in the main combustion chamber and the pre-chamber, which can capture the pressure change process and pressure oscillations phenomenon at the two positions, respectively. The result demonstrates that the throttling of the jet orifice leads to a significant three-stage pressure imbalance between the combustion chambers. With the reduction of the orifice diameter, the combustion acceleration of PJI is enhanced, resulting in an advanced combustion phase, improved combustion stability, and enhanced knock. The time-frequency analysis proves that the pressure oscillation propagation to the pre-chamber is frequency-selective and related to the orifice diameter. By matching the pre-chamber Helmholtz resonance frequency with the main chamber resonance frequency, strong pressure oscillations can be excited in the pre-chamber. Meanwhile, the pressure oscillation energy can be absorbed by the pre-chamber, which may help reduce the engine's combustion noise. Moreover, the PJI with an orifice diameter between 2 mm and 4 mm can improve the combustion stability with the ISFC reduced by 4.7–5.6%, and the IMEP increased by 1.2–2.6%.