The influence of dimethyl hydantoin (DMH) on the mechanical properties of GAP/RDX propellant was studied by molecular dynamics (MD) and dissipative particle dynamics (DPD) simulation. The results showed that the binding energies (E-binding) between GAP and different surfaces of RDX were in the order of (010)>(001)>(100). Compared to GAP/RDX, GAP grafted with DMH (GAP-DMH) exhibits higher binding energies with RDX, and the sequence of E-binding turns to (001)>(010)>(100). Radial distribution simulations demonstrated that GAP-DMH is more close to the surfaces of RDX, increasing the van der Waals energies between GAP-DMH and RDX. The stress and strain of GAP-DMH/RDX excel those of GAP/RDX. DPD simulations showed that GAP-DMH was able to restrain the agglomeration of RDX, to improve the dispersibility and to enlarge the contact surface with RDX, which also increased the mechanical properties of GAP/RDX propellant.