Hazards are the source of all kinds of accidents,no hazard no accident.In order to explore the rationale of accident-preventing,the paper analyzes the hazards systematically on the one hand,although hazards are countless even in types,it classifies them into three different levels according to their characteristic,i.e.,the energy source,the unsafe act & situation,and the causes of the unsafe act & situation.On the other hand,owing to the fact that the opposite of the causation of an accident is just the barrier to prevent it,based on the principle of defence-in-depth,this paper trace back to the causation of an accident,from its apparent causes to its root causes,from the root causes to the culture problems,and tries to find out the suitable barriers for the hazards of different levels.Through a systematical analysis of the hazards and its corresponding barriers,a hazard-barrier based safety risk management model is made.