Large eddy simulation is applied for the combustion process of n-heptane spray flame in methane environment. The influence of methane (CH4) concentration on the auto-ignition process and flame structures are discussed in detail. By increasing the CH4 concentration in the ambient gas, auto-ignition is delayed obviously at a low initial temperature because of the delayed first-stage ignition. While at a high temperature, the auto-ignition time is less influenced by the addition of CH4. However, after the addition of CH4, the concentration of the mixture where the high-temperature ignition is initialed becomes much richer and it is independent of the initial temperatures. In addition, the chemical explosive mode analysis (CEMA) method shows that the mixture in high-temperature regions is non-explosive without CH4 addition, and the abrupt transition between the explosive and non-explosive mixture at the tip of the reacting spray is observed by adding CH4 in the ambient gas, indicating that a flame front is developing downstream. This flame front exists for a long time at low CH4 concentration. The eigenvalue of the Jacobian matrix for the CH4/air mixture in the ambient gas is increased significantly with the addition of CH4, indicating that the combustion process is dominated by auto-ignition. Finally, results show that the development of the flame is accelerated by the addition of CH4 in the ambient gas.