Abstract: To address the problem of the poor inherent combustion characteristics of ammonia fuel， experimental fuels with different physicochemical characteristics were prepared using conventional highly active fuel — coal-to-liquid and fossil diesel， and the effects of the physicochemical characteristics of ignition fuel on the combustion characteristics in ammonia engines were deeply investigated. The results show that an appropriate increase in the cetane number of the ignition fuel is beneficial to the improvement of the mixture activity distribution in the cylinder， however， too high a cetane number will lead to insufficient premixed combustion， and the ignition effect will be weakened instead. Using high volatility fuel to ignite ammonia can mitigate the negative effects of increased ammonia ratio on the combustion process. Specifically， when the ammonia ratio increases from 50% to 70%， and ammonia is ignited by a fuel with a cetane number of 68.7 （I77， prepared by mixing 77% indirect coal-to-liquids diesel with 23% -35 ℃ civil fossil diesel according to volume ratio） and a fuel with a 50% distillation temperature（T₅₀） of 206.5 ℃ （direct coal-to-liquids， DCTL）， the indicated mean effective pressure（IMEP） only decreases by 7.46% and 3.58%， respectively. Notably， both ignition fuels can improve the ammonia combustion process， and the improvement effect is more significant at high ammonia ratios. Optical test results show that， at the 70% ammonia ratio， when ammonia is ignited by a fuel with a cetane number of 68.7 （I77） and a fuel with a 50% distillation temperature of 206.5 ℃ （DCTL）， the combustion flame can still propagate to the central area of the combustion chamber. Overall， compared with fuel volatility， adjusting the cetane number has a significant optimization effect on the combustion ammonia process. Therefore， in the design of ignition fuel， priority should be given to adjusting the cetane number of the ignition fuel， followed by fine-tuning in combination with volatility.