Abstract: With the development of modern weapon systems, the requirements for the survivability of ammunition in various complex environments have been continuously increasing. During the processes of storage, flight, and combat, ammunition may be subjected to extreme impact loads such as high-speed impacts, shock waves, bullet impacts, and fragment impacts, which may cause plastic deformation and fracture of the ammunition casing structure, and even detonate the internal explosives. Such processes involve complex phenomena such as impact, material thermomechanical coupling, chemical reactions of explosive, and explosions. Accurate prediction of the ammunition responses under impact is of great significance. Based on the Hot Optimal Transportation Meshfree (HOTM) method, a meshfree numerical approach was proposed to accurately predict the ammunition responses under different impact loadings. Coupled with EigenErosion method, the HOTM scheme enabled a robust simulation of highly-coupled thermomechanical problems involving large deformation, plasticity, phase transition, explosion, and material fracture. Meanwhile, a thermo-mechanical-chemical coupling constitutive model of explosives was established at macro scale, which took the effects of local temperature and local pressure on the explosive’s chemical reaction and detonation into account. Specifically, the Arrhenius thermo-chemical reaction kinetics model was applied to describe the dependence of explosive reactions on local temperature, and the Lee-Tarver pressure three-term ignition model was implemented to describe the dependence of detonation on local pressure. At the same time, the heat generated by the explosive reaction is also calculated via a chemical kinetics model. By coupling these models, the proposed model for explosives enables accurate simulations of the different detonation mechanisms of explosives at different impact velocities. With the help of the constitutive model, the HOTM method provides simulations of complex physical phenomena such as high-speed contact, large plastic deformation of the metal casing, material fracture, heat conduction, explosive detonation, and the expansion work of chemical reaction products during the process of ammunition being subjected to impact loads. To demonstrate the capability of the proposed method, two numerical simulations were conducted with different impact loadings, namely the bullet impacting the ammunition (at a speed of 850 m/s) and the fragment impacting the ammunition (at a speed of 1850 m/s). The simulation results show significant differences in the behaviors of explosives. Under low-velocity impact, only a part of the explosives experiences sharp reaction, implying the reaction is expedited due to high local temperature caused by large plastic deformation of metal casing and friction. As a contrast, the ammunition was detonated by high-velocity impact, which indicates that ultra-high pressure caused by the impact led to the significant reaction of the whole explosive and finally caused the detonation. The research results show the capability of the proposed approach to describe different ignition mechanisms under different loadings. Thus, the proposed approach can provide reliable technical support for the anti-impact design optimization and safety assessment of ammunition.