Abstract: Effects of start of ignition（SOI） and excess air coefficient（φ_a） on lean-burn combustion characteristics were studied on a pre-chamber marine natural gas engine.The main effect factors were analyzed quantitatively using Pareto method based on the experimental data.The results show that SOI is the major effect of the ignition delay and the heat release rate.After combustion optimization,the ignition delay can be shortened to less than 12°CA,and the heat release rate can be advanced to before 14°CA ATDC.φ_a acts as a more important role on the combustion duration.By controlling theφ_a between 1.66—1.84 at each load,the combustion duration can be less than 28°CA.The φ_a of this optimized pre-chamber marine natural gas engine is much higher than that of the counterpart for heavy-duty commercial vehicles.Since the pre-chamber has an independent gas supply,the pre-chamber marine natural gas engine can realize leaner combustion with higher brake mean effective pressure（BMEP） than the heavyduty natural gas engine,which is conducive to achieve higher thermal efficiency and lower NO_x emissions.