The deformation and damage process of POSS nanocomposite is investigated by molecule mechanics (MM) simulation. Firstly, the nano-scale models of two kinds of homopolymers, pure polystyrene (PS) and polystyrene attached with 5 mol% propyl-POSS (P-POSS-PS) were built. Then the mechanical behaviors of these two kinds of hybrid materials under focused uniaxial tensile loading and the remote uniaxial tensile loading are examined by MM simulations. It is found that a small quantity of POSS can observably increase the tensile modulus of the normal polymers. During tensile loadings, micro voids appear in the polymer matrix. With the increase of deformation, the micro voids become bigger and then connect to form the damage in bigger area. The POSS monomers prevent these micro voids from coalescence and thus retarding the formation of the damage. This would be helpful in understanding the reinforcement mechanism of POSS and provide important referential message for the applications of POSS.