The increasing importance of FlexRay systems in automotive domain inspires unceasing relative researches. One primary issue among researches is to verify the robustness of FlexRay systems either from protocol aspect or from system design aspect. However, research rarely addresses the issue that network topology selection will also cause difference in robustness. In this paper, we will illustrate how to model the robustness under various network topology adoptions by a well-known technology, fault tree analysis (FTA). Quantitative and qualitative analysis results can show the potential and suitability of FTA for modeling FlexRay systems' robustness.