In order to prevent any accident from happening, a safety assessment modeling approach combined with analytical hierarchy process (AHP), signed directed graph (SDG) and hazard and operability (HAZOP) is proposed for thermal power plants in this paper. AHP is used to divide the production equipments for thermal power plants into several hierarchies and it can simplify the modeling difficulties by using the hierarchy structure. SDG is applied to model cause and effect behavior of production equipments for thermal power plants. HAZOP analysis is implemented to locate the propagation paths of deviation to equipments, and a modeling technique for safety assessment, named hierarchical SDG-HAZOP, is constituted. This modeling approach can provide early warning information of accident for thermal power plants. Finally, the correctness and applicability of the proposed approach was testified successfully by a deaerator process.