Abstract: The space propulsive system is a power system composed of various types of engines.It must be capable of working in space for a long time with very high reliability.When a fault appears,it needs to be detected in time and shut off effectively.In the paper,the authors have introduced a fault reasoning method (fault directional graph) used in space propulsive system,and a corresponding fault reasoning model has been set up.Numerical simulation and theoretical analysis show that the model is simple and effective,and it is possible to be used for space application in a real time way.