There is significant interest in using pilot fuel as a source of ignition to enhance the performance of a natural gas engine. The pilot fuel injection timing is a critical factor that influences the combustion phasing and emissions characteristics. In this study, the effect of injection timing on the development of n-heptane spray flame in a methane/air environment is performed by using large eddy simulation (LES) in a constant-volume combustion vessel under engine-like conditions. The results indicate that injection retarding accelerates the ignition of n-heptane spray and reduces the interval between the ignition of n-heptane and methane/air mixture. The contributions of premixed and non-premixed combustion on heat release are distinguished by introducing the flame index (FI). Furthermore, the reason why retarding injection leads to earlier ignition is investigated by sensitivity analysis in a zero-dimension simulation. It is concluded that although methane addition delays the ignition of nheptane, the pre-ignition oxidative reactions of methane/air mixture occurs prior to the n-heptane injection, thereby increasing the temperature and pressure and producing significantly intermediate radicals and consequently promoting the n-heptane ignition. Additionally, the n-heptane spray flame, in turn, further ignites the methane/air mixture and induces the high-temperature combustion. The results also indicate that the influence of temperature on the ignition is maximum, which is followed by the influence of important intermediate species, formaldehyde (CH2O), while the influence of pressure is minor. The present study provides a theoretical basis for an in-depth understanding of the effect of injection timing on the combustion of dual-fuel engines.