Both effective identification and management of occupational safety risks require an unambiguous definition and a complete description to the hazard.The comparative analysis method was used to comb the definitions of a hazard in 7 standards and the factors to be considered in identifying hazards.The results of the analysis were used to study the content, classification and the relationship between the hazard and the hidden danger.The results show that the contents of a hazard have a very wide range.Energy, materials, human, human behaviors, the organization of humans, system, space and so on, all of these might be hazard, its classification strictly relies on accidents causation models, and that the hazard and the hidden danger are identical in the meaning.