Atomic oxygen undercutting effect of the protective polymer film in low earth orbit space environment is a potential threat to vulnerable long duration exposure facility. A Monte Carlo computational model is developed to simulate the interactions between atomic oxygen undercutting course with polyimide film. Physical process of the atomic oxygen undercutting, the definition of parameters, the affected by atomic oxygen fluence, orbit angle, protection coating thickness and thermal assimilation and the additive anti-undercutting components as well as a new three-dimensional reaction probability are discussed in detail. Simulated results are in good agreement with flight experimental data. With the increment of the atomic oxygen fluence, all the undercutting pro files, the depth and the width increased. Maximum undercutting depth is always larger than maximum undercutting width and the larger thermal assimilation coefficient causes the smaller undercutting damage. Using three-dimensional reaction probability, maximum depth decreased by approximately 20% than that of 28.5 degrees orbit angle. (C) 2010 Elsevier B. V. All rights reserved.