To overcome the disadvantage of experimental methods,plasticizer diffusion in the bond system of hydroxyl terminated polybutadiene (HTPB) propellant was simulated by the molecular dynamics (MD) method. The molecular models of plasticizer and the bond system were constructed in the software of Materials Studio 4.3. The mixture system performed geometry optimization was simulated under the condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field. The mean square displaces of plasticizer in the bond system were obtained by statistical average. The plasticizer diffusion was obtained by Einstein equation. The diffusion coefficients (10^-4 cm²·s^-1) of dioctyl sebacate (DOS) are respectively 0.0010,0.0020,0.0025,0.0031,0.0043 at the temperature of 273,298,310,323,348 K; and are respectively 0.0025,0.0020,0.0018,0.0015 at the content of 23%,37.5%,47%,60%. The results show that the diffusion coefficients gradually increase with increasing of environmental temperature and decrease a little with increasing of the DOS content.