A creep mechanism of PBX which is a polymer matrix composite filled by large number of solid particle is studied from the theory of deformation and sliding of molecular chain of the polymer in this paper. It is shown that the creep property of the polymer is the main factor influencing on the creep-damage properties of the composite though polymer content is so little in amount. The creep experimental results of Johnson are analyzed further by one-dimensional viscoelasticity-damage model about creep. The influence of the granularity on the creep-damage of PBX material and a certain probability of the creep-damage of PBX material are discussed initially.