Abstract: With the continuous increase in urban domestic waste, waste incineration power generation, driven by waste classification and technological progress, has become an important development direction for clean energy power generation due to its environmental and social benefits. Currently, mechanical grate incinerators account for more than 90% of waste-to-energy plants due to their high efficiency and stability. However, there is a lack of research on the impact of co-firing municipal solid waste (MSW) with leachate sludge on incinerator operations both domestically and internationally. Numerical simulations can play a significant role in studying the combustion state in a moving grate incinerator. In this study, taking a 700 t/d reverse-moving grate incinerator as the object, FLIC software and FLUENT software were employed respectively to simulate grate-firing on the moving grate and gas-phase combustion in the combustion chamber. By the numerical simulations, the temperature field and velocity field were obtained, as well as the concentration distributions of different gas components within the incinerator. The simulation results were compared with the actual operating data, and the maximum error did not exceed 10%, verifying the accuracy of the simulation. Based on this, the moving speed of the grate and the mass fraction of leachate sludge were optimized to improve the aerodynamic field and temperature field in the incinerator. Simulation results showed that reducing the moving speed of mechanical grate could improve the combustion state and combustion efficiency. The appropriate mass fraction of leachate sludge could reduce the local high temperature in the combustion chamber and reduce the slagging level caused by the high flue gas temperature near the secondary air nozzle of the rear wall. However, excessive mass fraction of leachate sludge would lead to the extension of the drying section, which made the high temperature zone closer to the rear wall, which is not conducive to the stable operation of the incinerator. Therefore, it is recommended to control the moving speed of mechanical grate at 3.56 m/h and maintain mass fraction of leachate sludge between 2% and 4%.